Musnad Abu Ya'la al-Mawsili 42:50

Abu al-Rabi' al-Zahrani narrated to us, Yahya bin Maymun—who was a companion of al-Mu'tamir—narrated to us, [he said] Ali bin Zayd narrated to us, from Abu Nadra, from Abu Sa'id, who said: A young man came to the Messenger of Allah (may Allah bless him and grant him peace) and said: "O Messenger of Allah, teach me a supplication by which I may attain good." He said to him: "Draw near." So he drew near until his knee almost touched the knee of the Messenger of Allah (may Allah bless him and grant him peace). He said: "Say: O Allah, pardon me, for You are the Pardoner, You love pardon, and You are the Generous Pardoner."