Musnad Ahmad 30:3818

Hashim narrated to us, Abu Mu'awiyah—meaning Shayban—narrated to us, from Layth, from Tawus, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware of suspicion, for it is the most untruthful of speech. Do not seek out the faults of others, do not spy on one another, do not envy one another, do not hate one another, do not compete against one another, and do not turn your backs on one another. And be, O servants of Allah, brothers as Allah has commanded you."