Musnad Ahmad 932:3

'Abd al-Rahman ibn Mahdi and Abu 'Amir narrated to us, they said: Zuhayr—meaning Ibn Muhammad—narrated to us, from 'Abdullah ibn Muhammad ibn 'Aqil, from al-Tufayl ibn Ubayy ibn Ka'b, from his father, from the Prophet (peace and blessings of Allah be upon him), who said: "My likeness among the prophets is like a man who built a house, and he made it beautiful and perfected it, but he left a space for a brick in it which he did not place. People began to walk around the structure, marveling at it and saying: 'If only this brick space were completed!' So, I am, among the prophets, the place of that brick."