Musnad Abu Ya'la al-Mawsili 6:35

Abu Khaythama narrated to us, Yahya bin Sa'id al-Qattan narrated to us, from Musa al-Juhani, he said: Mus'ab bin Sa'd narrated to me, from his father, from the Prophet (peace be upon him), he said: "Is one of you unable to earn one thousand good deeds every day?" A person from those sitting with him asked: "How does one of us earn one thousand good deeds?" He said: "He performs one hundred tasbihs (saying SubhanAllah), so he earns one thousand good deeds and one thousand bad deeds are removed from him."