Habits to be avoided, or corrected — the ones you are always criticizing in other people. You see, there’s profit to be derived everywhere; when you see or hear about some good example, you can imitate it, when you notice some habit that deserves censure, you can avoid it — or, if you’ve already got into it yourself, be at pains to correct it as soon as possible. Meanwhile, if you’ve a good eye for other people’s faults, don’t forget that they’re watching you.

[Illa etiam magis studeas cavere et vincere quæ tibi in aliis frequentius displicent. Ubique profectum tuum capies ut si bona exempla videas vel audias, de imitandis accendaris. Si quid autem reprehensibile confideraveris, cave ne idem facias, aut si aliquando fecisti, citius emendare te studeas. Sicut oculus tuus alios confiderat, sic iterum ab aliis notaris.]

Thomas à Kempis (c. 1380-1471) German-Dutch priest, author

The Imitation of Christ [De Imitatione Christi], Book 1, ch. 25, v. 4-5 (1.25.4-5) (c. 1418-27) [tr. Knox-Oakley (1959)]
    (Source)

See also Matthew 7:3-5.

(Source (Latin)). Alternate translations:

Study also to overcome in thyself those things that most mislike thee in other men, and take always some special profit in everyplace wheresoever thou come; as, if thou see any good example, enforce thee to follow it; and if thou see any evil example, look thou eschew it. As thy eye considereth the works of others, right so and in the same wise be thy works considered by others.
[tr. Whitford/Raynal (1530/1871)]

Study also to overcome in yourself those things that displease you most in others, and always gather some special profit from any place at all. For instance, if you see any good example, make yourself follow it, and if you see any evil example, see that you avoid it. As your eye considers the works of others, so and in the same manner your works are considered by others.
[tr. Whitford/Gardiner (1530/1955)]

Be carefull also to avoid with greater diligence those things in thy selfe which doe most displease thee in others. Gather some profit to thy soul out of every occasion whosoever thou be, so that if thou seest or hearest any good examples, be kindled and provoked to doe the like. But if thou perceive any thing worthy of reproofe, take heed of doing the same, or if thou have done it, labour quickly to amend it. As thou eyest other men so are other mens eies upon thee.
[tr. Page (1639), 1.25.16-19]

And, as this Knowledge of our Own Frailties and Necessities, it is greatly instrumental to our own Improvement, so we shall do well to profit by what we see in Others: and to be particularly concern'd for avoiding and subduing those Habits, which we find most usual and offensive in them with whom we Converse. For the Commonness of any Ill thing is so far from extenuating the Blame of those who copy after it, that it is the direct contrary; and such Examples should be lookt upon, as Marks which discover to us where the Rocks and Sands lie; such as are set to warn us off not to invite us in. Indeed a wise and good Man will turn Examples of all sorts, to his own Advantage. The Good he will make his Patterns, and strive to equal or excel them. The Bad he will by all means avoid; or if by Reflection the Deformity of his Neighbour's Actions happen to represent that of his own, he will be sure to do so no more; and think it a happy occasion, that he is thus grown Wiser by the Folly of others.
[tr. Stanhope (1696; 1706 ed.)]

Be particularly careful also to avoid chiefly those tempers and actions, that chiefly and most frequently displease thee in others. Wherever thou art, turn every thing to an occasion of improvement: if thou beholdeth or heareth of good examples, let them kindle in thee an ardent desire of imitation; if thou seest any thing blameable, beware of doing it thyself; or if thou hast done it, endeavor to amend it the sooner. As thy eye observeth, and thy judgment censureth others, so art thou observed and censured by them.
[tr. Payne (1803), 1.25.5]

Be careful also to avoid with great diligence those things in thyself, which do commonly displease thee in others. Gather some profit to thy soul wheresoever thou be; so as if thou seest or hearest of any good examples, stir up thyself to the imitation thereof. But if thou seest any thing worthy of reproof, beware thou do not the same. And if at any time thou hast done it, labour quickly to amend it. As thine eye observeth others, so art thou also noted again by others.
[ed. Parker (1841)]

Be careful also to avoid those things which more frequently displease thee on others. Wherever thou art, turn every thing to good account: if thou beholdest, or hearest of good examples, let them kindle in thee a desire of imitation; if thou observest any thing blameable , beware of doing it thyself; or, if thou hast occasionally done it, endeavour to amend it the sooner. As thy eye observeth others, so art thou observed by others in turn, and censured by them.
[tr. Dibdin (1851)]

Endeavour with greater zeal to guard against and conquer those vices which most frequently annoy thee in others. Turn all occasions to thy spiritual profit: so that if thou seest or hearest any good examples, thou mayest be spurred on to imitate them. But if thou observe anything that is blameworthy, take heed thou commit not the same; or if thou at any time hast done it, labour to correct thyself as soon as possible. As thine eye observeth others, so art thou in turn observed by others.
[ed. Bagster (1860)]

And strive also very earnestly to guard against and subdue those faults which displease thee most frequently in others. Gather some profit to thy soul wherever thou art, and wherever thou seest or hearest good examples, stir thyself to follow them, but where thou seest anything which is blameworthy, take heed that thou do not the same; or if at any time thou hast done it, strive quickly to amend thyself. As thine eye observeth others, so again are the eyes of others upon thee.
[tr. Benham (1874)]

Be careful also diligently to avoid those things in thyself, which do commonly displease thee in others. Gather some profit to thy soul wheresoever thou art; so that if thou seest any good examples, stir up thyself to the imitation thereof. But if thou observe anything worthy of reproof; beware thou do not the same. And if at any time thou hast done it, labour quickly to amend thyself. As thine eye observeth others, so art thou also noted again by others.
[tr. Anon. (1901)]

Also try more to avoid and overcome those things which most frequently displease you in others. Make your headway in every direction, so that, if you see or hear of good examples, you are fired to imitate them. But if you consider anything blameworthy, take care that you do not the same; or if at any time you have done so, quickly try to correct yourself. As your eye observes others, so you too are noted by them.
[tr. Daplyn (1952)]

Especially study to avoid and overcome those things that most displease you in other people. Strive to progress in all things, and let any examples you see or hear inspire you to imitate them. But if you observe anything blameworthy, take care not to do the same yourself. And should you ever have done so, amend your conduct without delay. As you observe others, so do others observe you.
[tr. Sherley-Price (1952)]

You must also take care to be on your guard and to overcome those things especially which displease you most often in others. You may find opportunities for progress everywhere -- if you see or hear of good examples, you should be inspired to follow them; if you observe anything that can be blamed, take care that you do not do the same. If you have ever done it, you must quickly mend your ways. As your own eye observes other people, you are in turn observed by them.
[tr. Knott (1962)]

Strive also to escape and overcome whatever you find offensive in others. May you grasp at every opportunity for progress. Where you see or hear good example may you be stirred to follow it. When you notice any deed worthy of condemnation, take care that you do not do likewise; and if you do so sometimes, try to correct yourself as quickly as possible. Just as you see others so also you are seen by them.
[tr. Rooney (1979)]