Sunan Ibn Majah Book 6, Hadith 1572

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدٍ، حَدَّثَنَا يَزِيدُ بْنُ كَيْسَانَ، عَنْ أَبِي حَازِمٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ زَارَ النَّبِيُّ ـ صلى الله عليه وسلم ـ قَبْرَ أُمِّهِ فَبَكَى وَأَبْكَى مَنْ حَوْلَهُ فَقَالَ: ‏ "‏ اسْتَأْذَنْتُ رَبِّي فِي أَنْ أَسْتَغْفِرَ لَهَا فَلَمْ يَأْذَنْ لِي وَاسْتَأْذَنْتُ رَبِّي فِي أَنْ أَزُورَ قَبْرَهَا فَأَذِنَ لِي فَزُورُوا الْقُبُورَ فَإِنَّهَا تُذَكِّرُكُمُ الْمَوْتَ ‏"‏ ‏.‏

It was narrated that Abu Hurairah said:“The Prophet (ﷺ) 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.’”