بُلُوغُ الْمَرَامِ
Bulugh al-Maram 224
The Book of Prayer
وَعَنْ عَلَيٍّ - رضى الله عنه - قَالَ : { كَانَ لِي مَعَ رَسُولِ اَللَّهِ - صلى الله عليه وسلم -مَدْخَلَانِ , فَكُنْتُ إِذَا أَتَيْتُهُ وَهُوَ يُصَلِّي تَنَحْنَحَ لِي } رَوَاهُ النَّسَائِيُّ , وَابْنُ مَاجَهْ . [1]
Narrated Ali (RA): "I had the permission of Allah's Messenger (ﷺ) to see him in his house at two times, and whenever I entered to him while he was praying he would clear his throat as a sign to me." [Reported by an-Nasa'i and Ibn Majah].
Sunnah.com reference: Book 2, Hadith 89