Abu Hurayrah (a Companion) narrated that the Prophet of Allah, sallallaahu 'alayhi wa sallam, visited his mother's grave, then he cried, and caused those around him to cry. He, sallallaahu 'alayhi wa sallam, said: "I asked permission from my Lord to seek forgiveness for her, but He did not allow me. Then I asked Him permission to visit her grave, so He allowed me that. So visit graves, for it reminds one of death."
he, sallallaahu 'alayhi wa sallam, cried is from sadness that she did not reach his time to believe in him.
Buraydah (a Companion) narrated that the Prophet of Allah, sallallaahu 'alayhi wa sallam, reached a marked grave, so he sat down and the people sat with him. He, sallallaahu 'alayhi wa sallam, moved his head as if he was speaking, then he cried. 'Umar ibn Al-Khattaab (a Companion; the second Caliph) said to him, "What is making you cry, O Messenger of Allah?
He, sallallaahu 'alayhi wa sallam, said: "This is the grave of Aaminah bint Wahb, I asked my Lord if I could visit her grave so He gave me permission, then I asked permission to seek forgiveness for her but He did not allow me, so I became sad for her and cried." Buraydah then said: “I never saw more people crying than at that time.”