Sahih Muslim Book 004, Hadith Number 2129.
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 Allah's Messenger, (may peace be upon him) as saying: I sought permission to beg forgiveness for my mother, but He did not grant it to me. I sought permission from Him to visit her grave, and He granted it (permission) to me.