Sayyiduna Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) visited the grave of his mother, and he (peace and blessings be upon him) wept and made those around him weep as well. Then he (peace and blessings be upon him) said: "I asked my Lord for permission to visit her grave, and I was granted permission; and I asked permission to pray for her forgiveness, and I was not granted permission. So visit the graves, for they remind of death."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 7157