Musnad Ahmad 788:81

Qutaybah ibn Sa'id narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from 'Amr—meaning Ibn Abi 'Amr—from al-Muttalib, from Abu Musa, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "Whoever performs a good deed and is pleased by it, and performs an evil deed and is saddened by it, then he is a believer."