Musnad Ahmad 30:367

Yazid narrated to us, Muhammad ibn Ishaq informed us, from Musa ibn Yasar, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "My likeness and the likeness of the prophets before me is like a man who built a structure, and he made it beautiful and completed it, except for the place of a brick in one of its corners. So people began to walk around it and marvel at it, and they said: 'We have not seen a structure more beautiful than this, except for the place of this brick.' And I was that brick."