Narrated Anas
Whenever the Prophet (ﷺ) asked permission to enter, he knocked the door thrice with greeting and whenever he spoke a sentence (said a thing) he used to repeat it thrice. (See Hadith No. 261, Vol. 8).
Narrated Anas
Whenever the Prophet (ﷺ) spoke a sentence (said a thing), he used to repeat it thrice so that the people could understand it properly from him and whenever he asked permission to enter, (he knocked the door) thrice with greeting.
Narrated `Abdullah bin `Amr
Once Allah's Messenger (ﷺ) remained behind us in a journey. He joined us while we were performing ablution for the `Asr prayer which was over-due. We were just passing wet hands over our feet (not washing them properly) so the Prophet (ﷺ) addressed us in a loud voice and said twice or thrice, "Save your heels from the fire."