Musnad Ahmad 32:238

Waki’ narrated to us, Hammad ibn Salamah narrated to us, from Abu al-Tayyah, from Anas ibn Malik, who said: The site of the Prophet’s (peace and blessings of Allah be upon him) mosque belonged to the Banu al-Najjar. It contained palm trees and the graves of polytheists. The Prophet (peace and blessings of Allah be upon him) said to them: "Name a price for it for me." They said: "We will not take a price for it." The Prophet (peace and blessings of Allah be upon him) was building it while they were handing him [the materials], and he was saying: "Indeed, the true life is the life of the Hereafter, so forgive the Ansar and the Muhajirun." He said: And the Messenger of Allah (peace and blessings of Allah be upon him) used to pray before the mosque was built wherever the prayer overtook him.