Musnad Ahmad 7:23

Yazid ibn Harun narrated to us, Mubarak ibn Fudalah informed us, al-Hasan narrated to us, he said: A man came to al-Zubayr ibn al-Awwam and said: "Shall I not kill Ali for you?" He said: "And how would you be able to kill him while the people are with him?" Then he mentioned its meaning.