أَخْبَرَنَا مُحَمَّدُ بْنُ قُدَامَةَ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنِ الْمُغِيرَةِ، عَنِ الْحَارِثِ الْعُكْلِيِّ، عَنْ أَبِي زُرْعَةَ بْنِ عَمْرِو بْنِ جَرِيرٍ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ نُجَىٍّ، عَنْ عَلِيٍّ، قَالَ كَانَ لِي مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم سَاعَةٌ آتِيهِ فِيهَا فَإِذَا أَتَيْتُهُ اسْتَأْذَنْتُ إِنْ وَجَدْتُهُ يُصَلِّي فَتَنَحْنَحَ دَخَلْتُ وَإِنْ وَجَدْتُهُ فَارِغًا أَذِنَ لِي ‏.‏

It was narrated that 'Ali said: "I had certain times when I used to come to the Messenger of Allah (PBUH). 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)." Translation Not Available

Book reference: Sunan An Nisai Book 13 Hadith 1211
Web reference: Sunan An Nisai Volume 2 Book 13 Hadith 1212