"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)." (Using translation from Nasāʾī 1211)
«كَانَتْ لِي مِنْ رَسُولِ اللهِ ﷺ سَاعَةٌ مِنَ السَّحَرِ آتِيهِ فِيهَا إِذَا أَتَيْتُهُ اسْتَأْذَنْتُ فَإِنْ وَجَدْتُهُ يُصَلِّي سَبَّحَ فَدَخَلْتُ وَإِنْ وَجَدْتُهُ فَارِغًا أَذِنَ لِي»