Hazrat Ibn Sirin narrates that Hazrat Zaid bin Thabit sought permission from Hazrat Usman to exhume the graves that were in Masjid Nabawi (peace and blessings be upon him), and he granted him permission. So he dug up those graves and removed them from the mosque (and buried them elsewhere), and those graves had been left in the mosque because there was very little soft land available for people.