Sahih Muslim Book 7, Hadith Number 3087.

Chapter : The wall of the Ka’ba and its door.

‘Aisha (Allah be pleased with her) reported: I asked Allah’s Messenger (may peace be upon him) about the wall, circumpassing the House (i.e. whether the wall on the side of Hijr was included in the Ka’ba). He said, Yes. I said: Then why did they not include it in the House? He said: ‘Your people ran short of the means (to do so). I said: Why is it that the level of its door is raised high? He said: Your people did it so that they should admit one whom they liked, and forbid him whom they disliked, and if your people were not new converts to faith, and I did not apprehend that their hearts would feel agitated at this. I would have definitely included (the area of) this wall-in the House and would have brought the door to the level of the ground.