Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 2129
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.