Sunan Ibn Majah 742

It was narrated that Anas bin Malik said:"The location where the Prophet's Mosque was built belonged to Banu Najjar. In it there were date-palm trees and graves of the idolaters. The Prophet said to them: 'Name its price.' They said: 'We will never take any money for it.' The Prophet built it and they were assisting him, and the Prophet was saying: 'The real life is the life of the Hereafter so forgive the Ansar and the Muhajirah.' Before the mosque was built, the Prophet would perform prayer wherever he was when the time for prayer came