Hadith #1213 (Sunan al-Nasa'i)
Book: The Book of Forgetfulness (In Prayer)
Narrator(s): al-Qasim bin Zakaria bin Dinar القاسم بن زكريا بن دينار القرشي, Hammad bin Usamah حماد بن أسامة - أبو أسامة, Shrhbyl bin Mdrk al-J'fy شرحبيل بن مدرك الجعفي الكوفي

“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).”

Source: https://parallelquran.com/hadith/sunan-al-nasai-1213/