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
When the Prophet (sallallahu alayhi wa sallam) 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.
Mishkat al-Masabih 200, Book 3, Knowledge, Hadith 11. 1 min read