سلسله احاديث صحيحه
Silsilat al-Ahadith al-Sahihah
الاداب والاستئذانBook of Etiquette and Seeking Permission
Repeating Three Times for Emphasis
Hadith 2812
ـ (كانَ إذا تَكلَّمَ بكلمَةٍ أعادَها ثلاثاً؛ حتّى تُفْهَمَ عنه، وإذا أتَى على قوْمٍ فَسَلَّمَ عليهم؛ سلّم عليهم ثلاثاً) .
It is narrated from Sayyiduna Anas (may Allah be pleased with him) that when the Noble Prophet (peace and blessings be upon him) would say something, he would repeat it three times until it was well understood, and when he would come to a people and greet them with salam, he would say salam three times as well.
