Musnad Ahmad 32:302

Yazid narrated to us, Hammad ibn Salamah informed 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, cultivated land, and graves from the graves of the pre-Islamic era (Jahiliyyah). The Messenger of Allah (peace and blessings of Allah be upon him) said to them: "Set a price for it for me." They said: "We do not seek a price for it except from Allah, the Almighty and Majestic." So the Messenger of Allah (peace and blessings of Allah be upon him) ordered the palm trees, and they were cut down; and the cultivated land, and it was cleared; and the graves, and they were exhumed. Before that, the Messenger of Allah (peace and blessings of Allah be upon him) used to pray in the sheep pens, wherever the prayer overtook him.