Al-Lu'lu Wal-Marjan 301 — The Book of Mosques and Places of Worship.

Anas Narrated from:

When the Prophet صلى الله عليه وسلم arrived at Al-Madîna, he dismounted at the upper side of Al-Madîna amongst the tribe called Banû ‘Amr bin ‘Aûf. He stayed there for fourteen nights. Then he sent for Banû An-Najjâr, and they came armed with their swords. As if I am looking (just now) as the Prophet صلى الله عليه وسلم was sitting on his Râhila (mount) with Abû Bakr riding behind him and all Banû An-Najjâr around him, till he dismounted at the courtyard of Abû Ayyûb’s house.The Prophet صلى الله عليه وسلم loved to offer Salât (prayer) wherever the time for As-Salât (the prayer) was due, even at sheep-folds. Later on he ordered that a mosque should be built and sent for some people of Banû An-Najjâr and said, “O Banû An-Najjâr! Suggest to me the price of this (walled) piece of land of yours." They replied, "No! By Allâh! We do not demand its price except from Allâh."Anas رضي الله عنه added: There were graves of pagans in it and some of it was unlevelled and there were some date-palm trees in it. The Prophet صلى الله عليه وسلم ordered that the graves of the pagans be dug out and the unlevelled land be levelled and the date-palm trees be cut down. (So all that was done).They aligned these cut date-palm trees towards the Qiblah of the mosque (as a wall), and they also built two stone side-walls (of the mosque). His companions brought the stones while reciting some poetic verses. The Prophet صلى الله عليه وسلم was with them and he kept on saying, "There is no goodness except that of the Hereafter, O Allâh! So please forgive the Ansâr (helpers), and Al-Muhajirah (the emigrants)."( Sahîh Al-Bukhârî, Hadîth No. 420, Vol. 1)