Sunan Ibn Majah, Volume 5, Book 33, Hadith 3708 (DarusSalam) ⓘ

It was narrated that Ali said: "I had two times of visiting the Messenger of Allah, at night and during the day. If I came to him when he was praying, he would clear his throat (to let me know he was praying)."

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا أَبُو بَكْرِ بْنُ عَيَّاشٍ، عَنْ مُغِيرَةَ، عَنِ الْحَارِثِ، عَنْ عَبْدِ اللَّهِ بْنِ نُجَىٍّ، عَنْ عَلِيٍّ، قَالَ كَانَ لِي مِنْ رَسُولِ اللَّهِ ـ صلى الله عليه وسلم ـ مُدْخَلاَنِ مُدْخَلٌ بِاللَّيْلِ وَمُدْخَلٌ بِالنَّهَارِ فَكُنْتُ إِذَا أَتَيْتُهُ وَهُوَ يُصَلِّي يَتَنَحْنَحُ لِي ‏.‏