Musnad Ahmad 1153:603

Qutaybah ibn Sa'id narrated to us, he said: 'Abd al-'Aziz ibn Muhammad narrated to us, from 'Alqamah ibn Abi 'Alqamah, from his mother, from 'Aishah, that she said: I used to love to enter the House (the Ka'bah) and pray inside it. So the Messenger of Allah, peace and blessings of Allah be upon him, took my hand and brought me into the Hijr. He said to me: "Pray in the Hijr if you wish to enter the House, for it is indeed a piece of the House. But your people fell short when they built the Ka'bah, so they excluded it from the House."