Musnad Ahmad 28:565

Hashim narrated to us, Ishaq—meaning Ibn Sa'id—narrated to us, Sa'id ibn 'Amr narrated to us, he said: 'Abdullah ibn 'Amr came to Ibn al-Zubayr while he was sitting in the Hijr, and said: "O Ibn al-Zubayr, beware of heresy (ilhad) in the sanctuary of Allah, for I bear witness that I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'A man from Quraysh will make it lawful and make it lawful in it; if his sins were weighed against the sins of mankind and jinn, they would outweigh them.'" He said: "Then take care that it is not you, O Ibn 'Amr, for you have read the scriptures and accompanied the Messenger of Allah, peace and blessings of Allah be upon him." He said: "Then I call you to witness that this is my face toward al-Sham, as a mujahid."