Reference for Psalms 86:15

וְאַתָּ֣ה אֲ֭דֹנָי אֵל־רַח֣וּם וְחַנּ֑וּן אֶ֥רֶךְ אַ֝פַּ֗יִם וְרַב־חֶ֥סֶד וֶאֱמֶֽת׃

But Thou, O Lord, art a God full of compassion and gracious, Slow to anger, and plenteous in mercy and truth.

ספר השרשים