Musnad Ahmad 32:1617

'Affan narrated to us, Hammad ibn Salamah narrated to us, from Abu al-Tayyah, from Anas ibn Malik, who said: The site of the Mosque of the Messenger of Allah (peace and blessings of Allah be upon him) belonged to the Banu al-Najjar. It contained crops, palm trees, and the graves of polytheists. He said: "O Banu al-Najjar, name a price for it to me." They said: "We do not seek a price for it except from Allah." He said: So he cut down the palm trees, leveled the crops, and exhumed the graves of the polytheists. He said: The Prophet of Allah (peace and blessings of Allah be upon him), before building the mosque, used to pray wherever the prayer overtook him, and in the sheepfolds. And the Prophet (peace and blessings of Allah be upon him) used to say while they were carrying the rocks for the building of the mosque: "O Allah, the true good is the good of the Hereafter, so forgive the Ansar and the Muhajirah."