Musnad Ahmad 30:3570

Rawh narrated to us, Malik narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Beware of suspicion, for suspicion is the most untruthful of speech. Do not seek out the faults of others, do not spy on one another, do not compete with one another (in worldly matters), do not envy one another, do not hate one another, and do not turn your backs on one another. And be, O servants of Allah, brothers."