Be at peace yourself. Then you will be able to bring peace to others.

[Pone te primus in pace, et tunc alios poteris pacificare.]

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

The Imitation of Christ [De Imitatione Christi], Book 2, ch. 3, v. 1 (2.3.1) (c. 1418-27) [tr. Rooney (1979)]
    (Source)

(Source (Latin)). Alternate translations:

First put thyself in peace, and then mayest thou the better pacify others.
[tr. Whitford/Raynal (1530/1871)]

First put yourself at peace, and then you may the better make others be at peace.
[tr. Whitford/Gardiner (1530/1955)]

Be first of all at peace with thy selfe, then maist thou be better able to pacifie others.
[tr. Page (1639)]

Secure Peace at Home in the first place; and, when thy own Breast is thus composed, it will then be proper to Reconcile and make Peace among thy Neighbors.
[tr. Stanhope (1696; 1706 ed.)]

Thou must first secure the peace of thy own breast; before thou wilt be qualified to restore peace to others.
[tr. Payne (1803)]

First, keep thyself in peace, and then shalt thou be able to pacify others.
[ed. Parker (1841)]

Thou must first secure the peace of thy own breast ere thou be qualified to restore peace to others.
[tr. Dibdin (1851)]

First keep thyself in peace, and then thou wilt be able to bring others to peace.
[ed. Bagster (1860)]

First keep thyself in peace, and then shalt thou be able to be a peacemaker towards others.
[tr. Benham (1874)]

First keep thyself in peace, and then thou shalt be able to keep peace among others.
[tr. Anon. (1901)]

First keep peace with yourself; then you will be able to bring peace to others.
[tr. Croft/Bolton (1940)]

Keep yourself at peace first, and then you will be able to make others peaceful.
[tr. Daplyn (1952)]

Firstly, be peaceful yourself, and you will be able to bring peace to others.
[tr. Sherley-Price (1952)]

Peace in your own soul first of all, then you can think about making peace between other people.
[tr. Knox-Oakley (1959)]

Live in peace yourself and then you can bring peace to others.
[tr. Knott (1962)]

Keep yourself at peace first, and then you will be able to bring peace to others.
[tr. Creasy (1989)]