Don’t let the future disturb you. You will meet it, if you have to, with the same weapons of reason which today arm you against the present.

[Τὰ μέλλοντα μὴ ταρασσέτω· ἥξεις γὰρ ἐπ᾿ αὐτά, ἐὰν δεήσῃ, φέρων τὸν αὐτὸν λόγον, ᾧ νῦν πρὸς τὰ παρόντα χρᾷ.]

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher

Meditations [To Himself; Τὰ εἰς ἑαυτόν], Book 7, ch. 8 (7.8) (AD 161-180) [tr. Coker (2022)]
    (Source)

(Source (Greek)). Alternate translations:

Let not things future trouble thee. For if necessity so require that they come to pass, thou shalt (whensoever that is) be provided for them with the same reason, by which whatsoever is now present, is made both tolerable and acceptable unto thee.
[tr. Casaubon (1634), 7.6]

Be not disturb'd about the Future; for if ever you come to it, you'll have the same Reason for your Guide, and Protection, which preserves you at present.
[tr. Collier (1701)]

Be not disturbed about futurity: You shall come to encounter with future events, possessed of the same reason you now employ in your present affairs.
[tr. Hutcheson/Moor (1742)]

Be not solicitous about future possibilities. You will encounter them when they approach, under the conduct of the same reason which you make use of on every present emergency.
[tr. Graves (1792)]

Let not future things disturb thee, for thou wilt come to them, if it shall be necessary, having with thee the same reason which now thou usest for present things.
[tr. Long (1862)]

Be not disturbed about the future, for if ever you come to it, you will have the same reason for your guide, which preserves you at the present.
[tr. Collier/Zimmern (1887)]

Let not the future perturb you. You will face it, if so be, with the same reason which is yours to meet the present.
[tr. Rendall (1898)]

Be not troubled about the future. You will come to it, if need be, with the same power to reason, as you use upon your present business.
[tr. Hutcheson/Chrystal (1902)]

Be not disquieted about the future. If thou must come thither, thou wilt come armed with the same reason which thou appliest now to the present.
[tr. Haines (Loeb) (1916)]

Let not the future trouble you; for you will come to it, if come you must, bearing with you the same reason which you are using now to meet the present.
[tr. Farquharson (1944)]

Never let the future disturb you. You will meet it, if you have to, with the same weapons of reason which today arm you against the present.
[tr. Staniforth (1964)]

Do not allow the future to trouble your mind; for you will come to it, if come you must, bringing with you the same reason that you now apply to the affairs of the present.
[tr. Hard (1997 ed.)]

Forget the future. When and if it comes, you’ll have the same resources to draw on -- the same logos.
[tr. Hays (2003)]

Do not let the future trouble you. You will come to it (if that is what you must) possessed of the same reason that you apply now to the present.
[tr. Hammond (2006)]

Do not allow the future to trouble your mind; for you will come to it, if come you must, bringing with you the same reason that you now apply to the affairs of the present.
[tr. Hard (2011 ed.)]