Musnad Ahmad 5:336

Ali ibn Ishaq narrated to us, Abdullah informed us, Yahya ibn Ayyub informed us, from Ubayd Allah ibn Zahr, from Ali ibn Yazid, from al-Qasim, from Abu Umamah, that Ali ibn Abi Talib informed him that: He used to come to the Prophet, peace and blessings of Allah be upon him. He said: "When I found him praying, he would glorify [Allah] and I would enter. And when he was not praying, he would give me permission [to enter]."