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 (RU)
English reference (RU): Book 33, Hadith 214 (RU)
Arabic reference (RU): Book 34, Hadith 224 (RU)