Musnad Ahmad 5:9

Abu Sa’id narrated to us, ‘Abd al-Wahid ibn Ziyad al-Thaqafi narrated to us, ‘Umarah ibn al-Qa’qa’ narrated to us, from al-Harith ibn Yazid al-‘Ukli, from Abu Zur’ah, from ‘Abdullah ibn Naji, who said: ‘Ali said: "I had an hour at the time of the dawn (al-sahar) in which I would enter upon the Messenger of Allah, may Allah bless him and grant him peace. If he was standing praying, he would glorify Allah (tasbih) for me, and that was his permission for me. And if he was not praying, he would give me permission."