Psalm 18:25 in the World English Bible
With the merciful you will show yourself merciful. With the perfect man, you will show yourself perfect.
Context
24 Therefore hath the LORD recompensed me according to my righteousness, according to the cleanness of my hands in his eyesight.
25 With the merciful thou wilt shew thyself merciful; with an upright man thou wilt shew thyself upright;
26 With the pure thou wilt shew thyself pure; and with the froward thou wilt shew thyself froward.