Musnad Abu Ya'la al-Mawsili 146:7

Zuhayr ibn Harb narrated to us, Yazid narrated to us, Hammad ibn Salamah narrated to us, from Abu al-Tayyah, from Anas, who said: The site of the Mosque of the Prophet, may Allah bless him and grant him peace, 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, may Allah bless him and grant him peace, said to them, "Name a price for it to me." They said, "We do not seek a price for it except from Allah." He said: So the Messenger of Allah, may Allah bless him and grant him peace, ordered the palm trees, and they were cut down; and the cultivated land, and it was cleared; and the graves, and they were exhumed. The Messenger of Allah, may Allah bless him and grant him peace, used to pray before that in the sheep pens and wherever the prayer overtook him.