Sahih al-Bukhari 95
Sahih al-Bukhari · Book 3: Knowledge · Repeating ones talk thrice in order to make others understand
Narrated Anas:
حَدَّثَنَا عَبْدَةُ بْنُ عَبْدِ اللَّهِ ، 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 تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلاَثًا حَتَّى تُفْهَمَ عَنْهُ، وَإِذَا أَتَى عَلَى قَوْمٍ فَسَلَّمَ عَلَيْهِمْ سَلَّمَ عَلَيْهِمْ ثَلاَثًا.
Whenever the Prophet (ﷺ) spoke a sentence (said a thing), he used to repeat it thrice so that the people could understand it properly from him and whenever he asked permission to enter, (he knocked the door) thrice with greeting.