The Prophet ﷺ 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 ﷺ ordered the graves of the pagans to be dug out and the land to be leveled, and the date-palm trees to be cut down. The cut datepalms were fixed in the direction of the Qibla of the mosque.  

عَنْ أَنَسٍ ؓ قَدِمَ النَّبِيُّ ﷺ الْمَدِينَةَ فَأَمَرَ بِبِنَاءِ الْمَسْجِدِ فَقَالَ يَا بَنِي النَّجَّارِ ثَامِنُونِي فَقَالُوا لاَ نَطْلُبُ ثَمَنَهُ إِلاَّ إِلَى اللَّهِ فَأَمَرَ بِقُبُورِ الْمُشْرِكِينَ فَنُبِشَتْ ثُمَّ بِالْخِرَبِ فَسُوِّيَتْ وَبِالنَّخْلِ فَقُطِعَ فَصَفُّوا النَّخْلَ قِبْلَةَ الْمَسْجِدِ