Sunan Ibn Majah Book 25, Hadith 74

“I asked the Messenger of Allah (ﷺ) about the Hijr, and he said: ‘It is part of the House.’ I said: ‘What kept them from incorporating it into it?’ He said: ‘They ran out of funds.’ I said: ‘Why is its door so high up that it can only be reached with a ladder?’* He said: ‘That is what your people did so that they could let in whoever they wanted and keep out whoever they wanted. Were it not that your people have so recently left disbelief behind, and I am afraid that it would bother them, I would have changed it, incorporating what they left out and I would put its door at ground level.”