Abu Musa Al-Ash'ari (May Allah be pleased with him) reported:
The Messenger of Allah ﷺ said, "Permission is to be sought thrice. If it is accorded, you may enter; otherwise, go back."

[Al-Bukhari and Muslim].

وعن أبي موسي الأشعري رضي الله عنه قال: قال رسول الله ﷺ : “ الاستئذان ثلاث، فإن أذن لك وإلا فارجع" ((متفق عليه))