Musnad Ahmad 30:984

Yahya ibn Adam narrated to us, Sufyan narrated to us, from Dawud ibn Qays, from Abu Sa’id, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The Muslim is the brother of the Muslim; he does not wrong him, he does not forsake him, and he does not belittle him. It is sufficient evil for a man that he belittles his Muslim brother."