Musannaf Ibn Abi Shayba 33:478

Abu Khalid narrated to us, from Dawud, from Ali ibn Zayd, from Abu Uthman, who said: It reached me from Abu Hurayrah that he said: "Indeed, Allah rewards the believer for a single good deed with a million good deeds." So I went to him and said: "O Abu Hurayrah, it has reached me that you say that Allah rewards the believer for a single good deed with a million good deeds." He said: "Yes, and two million good deeds. And there is in the Qur'an [evidence] for that: 'Indeed, Allah does not do injustice, [even] as much as an atom's weight; while if there is a good deed, He multiplies it' (4:40). So who knows the naming of those multiplications? 'And gives from Himself a great reward' (4:40)." He said: "It is Paradise."