It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I asked permission to pray for forgiveness for my mother, but permission was not given; then permission was given (to me) to visit her grave."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14079