Musnad Abu Ya'la al-Mawsili 155:353

Ahmad ibn Mani' narrated to us, Marwan ibn Mu'awiyah narrated to us, from Yazid ibn Kaysan, from Abu Hazim, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I sought permission from my Lord to ask for forgiveness for my mother, but He did not grant me permission. And I sought His permission to visit her grave, and He granted me permission."