Musnad Ahmad 5:205

Yahya ibn Adam narrated to us, Ibn al-Mubarak narrated to us, from Yahya ibn Ayyub, from Ubayd Allah ibn Zahr, from Ali ibn Yazid, from al-Qasim, from Abu Umamah, from Ali, who said: "When I would seek permission to enter upon the Messenger of Allah (may Allah bless him and grant him peace), if he was in prayer, he would say 'Subhan Allah' (glorify Allah), and if he was otherwise, he would grant me permission."