Musnad Ahmad 30:827

Yazid narrated to us, Mubarak ibn Fudalah informed us, from Ali ibn Zayd, from Abu Uthman al-Nahdi, who said: I came to Abu Hurayrah and said to him: "It has reached me that you say: 'Indeed, a good deed is multiplied by one million good deeds.'" He said: "And what is it that surprises you about that? For by Allah, I heard it—meaning from the Prophet (peace and blessings of Allah be upon him)." Abdullah ibn Ahmad said: "My father said it like this: 'Indeed, Allah multiplies a good deed by two million good deeds.'"