(13) باب التَّسْلِيمِ وَالاِسْتِئْذَانِ ثَلاَثًا
Narrated Anas:
Whenever Allah's Messenger (ﷺ) greeted somebody, he used to greet him three times, and if he spoke a sentence, he used to repeat it thrice.
Reference: Sahih al-Bukhari 6244
In-book reference: Book 79, Hadith 18
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 74, Hadith 261