Musnad Ahmad 26:259

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Mansur, from Abu Wa'il, from 'Abdullah ibn Mas'ud, who said: A man said to the Messenger of Allah, peace and blessings of Allah be upon him: "How can I know if I have done good, and if I have done evil?" The Prophet, peace and blessings of Allah be upon him, said: "If you hear your neighbors saying: 'You have done good,' then you have done good; and if you hear them saying: 'You have done evil,' then you have done evil."