Musnad Ahmad 30:2560

Muhammad ibn Ubayd al-Tanafisi narrated to us, he said: Yazid ibn Kaysan narrated to us, from Abu Hazim, from Abu Hurayrah, who said: The Prophet, peace and blessings of Allah be upon him, visited the grave of his mother, and he wept, and those around him wept. The Messenger of Allah, peace and blessings of Allah be upon him, said: "I sought permission from my Lord to ask for forgiveness for her, but I was not permitted. And I sought His permission to visit her grave, and He permitted me. So visit the graves, for they remind one of death."