Musnad Ahmad 30:997

Abu al-Qasim, may Allah bless him and grant him peace, said: "My likeness and the likeness of the prophets before me is like a man who built houses, and he made them excellent, complete, and beautiful, except for the place of a brick in one of its corners. So people began to walk around it, and the structure amazed them, and they said: 'Would that a brick were placed here, so that your structure would be completed.' Muhammad the Prophet, may Allah bless him and grant him peace, said: 'So I was that brick.'"