Sahih Muslim Book 4, Hadith Number 2130.

Chapter : The Apostle of Allah (way peace be upon him) seeking permission from the Lord. the Exalted and High, for visiting the grave of his mother.

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.