Musnad Ahmad 788:72

Abu Nu'aym narrated to us, Talhah ibn Yahya ibn Talhah narrated to us, from Abu Burdah, from Abu Musa, who said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "Let one of you seek permission three times. If permission is granted to him, then [he may enter], otherwise, let him return."