Musannaf Ibn Abi Shayba 13:2720

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn ‘Awn, who said: I was with Mujahid in the market of Kufa, and there were tents for the tailors facing the market from the side of the houses of the mourners. He said: Ibn ‘Umar used to seek permission in [a situation] like this. He said: And I said, "How would he do?" He said: He would say, "Peace be upon you, may I enter?" Then he would enter.