(17) باب التَّنَحْنُحِ فِي الصَّلاَةِ
It was narrated that 'Ali said:
"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)."
Reference: Sunan an-Nasa'i 1211
In-book reference: Book 13, Hadith 33
English translation: Vol. 2, Book 13, Hadith 1212