Anas (May Allah be pleased with him) reported:
Whenever the Prophet (ﷺ) said something, he would repeat his words thrice so that the meanings would be understood fully; and whenever he came upon a group of people, he would greet them, and he would repeat salutation thrice.
[Al-Bukhari and Muslim]. (PT)
App reference (PT): Book 2, Hadith 16 (PT)
Arabic/English book reference (PT): Book 2, Hadith 696 (PT)