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: 5708
Anas b. Milik reported Allah's Messenger (may peace be upon him) as saying: There would be such a vast distance between the sides of my Cistern as it is between Sana' and Medina.