Hadith 1212
Chapter 17: Clearing the throat while praying - كتاب السهو
أَخْبَرَنِي مُحَمَّدُ بْنُ عُبَيْدٍ، قَالَ حَدَّثَنَا ابْنُ عَيَّاشٍ، عَنْ مُغِيرَةَ، عَنِ الْحَارِثِ الْعُكْلِيِّ، عَنِ ابْنِ نُجَىٍّ، قَالَ قَالَ عَلِيٌّ كَانَ لِي مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم مَدْخَلاَنِ مَدْخَلٌ بِاللَّيْلِ وَمَدْخَلٌ بِالنَّهَارِ فَكُنْتُ إِذَا دَخَلْتُ بِاللَّيْلِ تَنَحْنَحَ لِي .
It was narrated that 'Ali said:
"I had two times when I would enter upon the Messenger of Allah (ﷺ), one at night and one during the day. When I entered at night he would clear his throat (to tell me to come in)."
Reference : Sunan an-Nasa'i 1212
In-book reference : Book 13, Hadith 34