Sahih Muslim
The Book Pertaining to the Excellent Qualities of the Holy Prophet (may Peace be upon them) and His Companions (Kitab Al-Fadail)
Book Number: 030 / Hadith Number: 5673
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: The similitude of mine and that of the Apostles (before me) is that of a person who constructed a building and he built it fine and well and the people went round it saying: Never have we seen a building more imposing than this. but for one brick, and I am that brick (with which you give the finishing touch to the building).