The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 2130
Abu Huraira reported: The Apostle of Allah (may peace be upon him) visited the grave of his mother and he wept, and moved others around him to tears, and said: I sought permission from my Lord to beg forgiveness for her but it was not granted to me, and I sought permission to visit her grave and it was granted to motel So visit the graves, for that makes you mindful of death.