Hammad bin Usamah حماد بن أسامة - أبو أسامة · The Book of Forgetfulness (In Prayer)

"I had certain times when I used to come to the Messenger of Allah (ﷺ). When I came to him I would ask for permission to enter. If I found him praying he would clear his throat and I would enter, and if I found him free he would give me permission (to enter)."