Musnad Ahmad 30:740

Husayn ibn Ali al-Ju'fi narrated to us, from Za'idah, from Abdullah ibn Dhakwan, from Abd al-Rahman al-A'raj, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Beware of suspicion, for suspicion is the most untruthful of speech. Do not spy, do not pry, do not compete [in worldly matters], do not outbid one another [to deceive], do not turn your backs on one another, and do not hate one another. And be, O servants of Allah, brothers."