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 (PBUH), one at night and one during the day. When I entered at night he would clear his throat (to tell me to come in)." Translation Not Available

Book reference: Sunan An Nisai Book 13 Hadith 1212
Web reference: Sunan An Nisai Volume 2 Book 13 Hadith 1213