"The Messenger of Allah visited the grave of his mother and wept, and caused those around him to weep. He said: 'I asked my Lord for permission to pray for forgiveness for her and He did not give me permission, and I asked Him for permission to visit her grave and He gave me permission, so visit the graves, for they will remind you of death."'

Sunan an-Nasa'i Book 21, Hadith 217