In one report the words are: "I came to Sayyiduna Abu Hurayrah (may Allah be pleased with him) and said: 'I have learned that you say: a single good deed becomes 1000 thousand (i.e. a million times).' Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: 'Why are you surprised at this? By Allah, I heard this from the tongue of the Prophet (peace and blessings be upon him).'" After this the narrator mentioned the report as before.
Ahmad recorded it with two chains, and al-Bazzar recorded something similar; the narrators of one of Ahmad's two chains are good.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأذكار / 17189