Sahih al-Bukhari 94
Sahih al-Bukhari · Book 3: Knowledge · Repeating ones talk thrice in order to make others understand
Narrated Anas:
حَدَّثَنَا عَبْدَةُ ، قَالَ حَدَّثَنَا عَبْدُ الصَّمَدِ ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْمُثَنَّى ، قَالَ حَدَّثَنَا ثُمَامَةُ بْنُ عَبْدِ اللَّهِ ، عَنْ أَنَسٍ ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ كَانَ إِذَا سَلَّمَ سَلَّمَ ثَلاَثًا، وَإِذَا تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلاَثًا.
Whenever the Prophet (ﷺ) asked permission to enter, he knocked the door thrice with greeting and whenever he spoke a sentence (said a thing) he used to repeat it thrice. (See Hadith No. 261, Vol. 8).