Sahih Bukhari Volume 3, Book 30, Hadith Number 92.
Narated By Anas : The Prophet came to Medina and ordered a mosque to be built and said, “O Bani Najjar! Suggest to me the price (of your land).” They said, “We do not want its price except from Allah” (i.e. they wished for a reward from Allah for giving up their land freely). So, the Prophet ordered the graves of the pagans to be dug out and the land to be levelled, and the date-palm trees to be cut down. The cut date-palms were fixed in the direction of the Qibla of the mosque.