Musnad Ahmad 8:58

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu 'Abdullah, the freed slave of Juhaynah, who said: I heard Mus'ab ibn Sa'd narrating from Sa'd, from the Messenger of Allah (may Allah bless him and grant him peace) that he said: "Is one of you unable to earn one thousand good deeds in a day?" Someone asked: "Who can endure that?" He said: "He performs one hundred tasbihs (saying 'SubhanAllah'), and one thousand good deeds are written for him, and one thousand bad deeds are erased from him."