Sunan Ibn Majah, Chapter: Chapters Regarding Funerals

"The Prophet (saw) visited the grave of his mother and wept, causing the people around him to weep. Then he said: 'I asked my Lord for permission to seek forgiveness for her, but He did not give me permission. Then I asked my Lord for permission to visit her grave and He gave me permission. So visit the graves, for they will remind you of death.'" Hasan