Musnad Ahmad 1018:102

Abu Kamil narrated to us, Zuhayr narrated to us, Hisham ibn Urwah narrated to us, from his father, from al-Ahnaf ibn Qays, from a paternal uncle of his, that he came to the Messenger of Allah, may Allah bless him and grant him peace, and said: "Tell me a statement that will benefit me, and make it brief, so that I may perhaps comprehend it." He said: "Do not become angry." He returned to him repeatedly, and each time the Messenger of Allah, may Allah bless him and grant him peace, would return to him [with the same answer]: "Do not become angry."