Psalm 18:25
23 I was innocent before him, and kept myself from sinning. 24 The Lord rewarded me for my godly deeds; he took notice of my blameless behavior. 25 You prove to be loyal to one who is faithful; you prove to be trustworthy to one who is innocent. 26 You prove to be reliable to one who is blameless, but you prove to be deceptive to one who is perverse. 27 For you deliver oppressed people, but you bring down those who have a proud look.
Matthew 5:7
5 Blessed are the meek, for they will inherit the earth. 6 Blessed are those who hunger and thirst for righteousness, for they will be satisfied. 7 Blessed are the merciful, for they will be shown mercy. 8 Blessed are the pure in heart, for they will see God. 9 Blessed are the peacemakers, for they will be called the children of God.
Notes and References
"... 'Blessed are the merciful, for they shall obtain mercy.' How should one understand 'the merciful'? Consider the following examples. The Masoretic text of Psalm 18:25-26 reads: "with the merciful you show yourself merciful." In b. Baba Qamma 9:30 there is a nearly identical statement ("So long as you are merciful, He will have mercy on you"). Similar sayings are also found in Sifre Deuteronomy 13.18 and b. Shabbat 151b. Early Christian comments that are similar are found in 1 Clement 13:2 ("Be merciful in order that you might receive mercy") and James 2:13 ("judgment is without mercy to one who has shown no mercy"). Similar sentiments are echoed in Matthew 18:33; 9:13; 12:7; 15:22; 17:15; and 20:30-31 ..."
Talbert, Charles H. Reading the Sermon on the Mount: Character Formation and Decision Making in Matthew 5-7 (p. 52) University of South Carolina Press, 2004