Musnad Ahmad 1018:76

Husayn ibn Muhammad narrated to us, Ibn Abi al-Zinad narrated to us, from his father, from Urwah, from al-Ahnaf ibn Qays, who said: A cousin of mine informed me, saying: I said to the Messenger of Allah, may Allah bless him and grant him peace: "O Messenger of Allah, tell me a statement and make it brief, perhaps I may comprehend it." He said: "Do not get angry." He said: So I returned to him repeatedly, and each time the Messenger of Allah, may Allah bless him and grant him peace, would return to me: "Do not get angry."