Etiquette of Seeking Permission 

`Abdullah ibn Busr (may Allah be pleased with him) narrated: When the Messenger of Allah (peace be upon him) came to some people’s door, he did not face it squarely, but faced the right or left corner, and said: Peace be upon you! peace be upon you! That was because there were no curtains on the doors of the house at that time. (Abu Dawud)