ASK PERMISSION TO ENTER H 883 "Permission to enter is to be sought three times. Then if permission is granted you may enter; otherwise return." R 883 Bukhari and Muslim with a chain up to Abu Musa Ash'ari who related that the Prophet, praise and peace be upon him, said this.
870- عن أَبي موسى الأشعري رضي الله عنه ، قَالَ : قَالَ رسول الله صلى الله عليه وآله وسلم : "الاسْتِئْذَانُ ثَلاثٌ ، فَإنْ أُذِنَ لَكَ وَإِلاَّ فَارْجِعْ" متفقٌ عَلَيْهِ .