Mishkat al-Masabih 2 Knowledge كتاب العلم 11

وَعَنْ أَنَسٍ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا تَكَلَّمَ بِكَلِمَةٍ أَعَادَهَا ثَلَاثًا حَتَّى تُفْهَمَ عَنْهُ وَإِذَا أَتَى عَلَى قَوْمٍ فَسَلَّمَ عَلَيْهِمْ سَلَّمَ عَلَيْهِمْ ثَلَاثًا “. رَوَاهُ الْبُخَارِيُّ

Anas said that when the Prophet made a statement he repeated it three times so that it would be understood, and that when he met a company and gave them a salutation he did it three times. Bukhari transmitted it.