Abu Dawud Book 2, Hadith Number 1517.
Chapter : On seeking pardon and repentance.
Narated By Mu’adh b. Jabal : The Apostle of Allah (PBUH) caught his hand and said: By Allah, I love you, Mu’adh. I give some instruction to you. Never leave to recite this supplication after every (prescribed) prayer: “O Allah, help me in remembering Thee, in giving Thee thanks, and worshiping Thee well.”