Whenever God's Prophet (a.s) went to someone's door, he would not leave until he greeted the residents of the house three times so as to seek permission to enter.
كان رسول الله (صلى الله عليه وآله) إذا أتى باب قوم لم ينصرف حتى يؤذن بالسلام ثلاث مرات.