Sahih al-Bukhari, Hadith 94 ⓘ
Narrated Anas: Whenever the Prophet 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).
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.
also referenced under: Book 3, Hadith 36 (In-Book) & Volume 1, Book 3, Hadith 94 (USC-MSA)