Musnad Ahmad 5:37

'Abdullah narrated to us, Abu Kurayb Muhammad ibn al-'Ala' narrated to me, Ibn al-Mubarak narrated to us, from Yahya ibn Ayyub, from 'Ubaydullah ibn Zahr, from 'Ali ibn Yazid, from al-Qasim, from Abu Umamah, who said: 'Ali said: "I used to come to the Prophet, may Allah bless him and grant him peace, and seek permission. If he was in prayer, he would glorify Allah (say 'SubhanAllah'), and if he was not in prayer, he would give me permission."