Ash-Shama'il Al-Muhammadiyah, Book 33, Hadith 214 (English)

Anas ibn Maalik radiyallahu anhu says, Rasoolullah sallallahu alaihe wasallam (sometimes) repeated a word (as was necessary) thrice, so that his listeners understood well what he was saying

حَدَّثَنَا مُحَمَّدُ بْنُ يَحْيَى، قَالَ‏:‏ حَدَّثَنَا أَبُو قُتَيْبَةَ سَلْمُ بْنُ قُتَيْبَةَ، عَنْ عَبْدِ اللهِ بْنِ الْمُثَنَّى، عَنْ ثُمَامَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ‏:‏ كَانَ رَسُولُ اللهِ صلى الله عليه وسلم، يُعِيدُ الْكَلِمَةَ ثَلاثًا لِتُعْقَلَ عَنْهُ‏.‏

also referenced under: Book 34, Hadith 224 (In-Book)