I heard Ash-Shuʿbi saying, "A person who was accompanying the Prophet ﷺ passed by a grave that was separated from the other graves told me that the Prophet ﷺ once led the people in the (funeral) prayer and the people had aligned behind him. I said, "O Aba ʿAmr! Who told you about it?" He said, "Ibn ʿAbbas."  

أَخْبَرَنِي مَنْ مَرَّ مَعَ النَّبِيِّ ﷺ عَلَى قَبْرٍ مَنْبُوذٍ فَأَمَّهُمْ وَصَفُّوا عَلَيْهِ فَقُلْتُ يَا أَبَا عَمْرٍو مَنْ حَدَّثَكَ فَقَالَ ابْنُ عَبَّاسٍ  

A black person, a male or a female used to clean the Mosque and then died. The Prophet ﷺ did not know about it . One day the Prophet ﷺ remembered him and said, "What happened to that person?" The people replied, "O Messenger of Allah ﷺ! He died." He said, "Why did you not inform me?" They said, "His story was so and so (i.e. regarded him as insignificant)." He said, "Show me his grave." He then went to his grave and offered the funeral prayer.  

أَنَّ أَسْوَدَ رَجُلاً أَوِ امْرَأَةً كَانَ يَقُمُّ الْمَسْجِدَ فَمَاتَ وَلَمْ يَعْلَمِ النَّبِيُّ ﷺ بِمَوْتِهِ فَذَكَرَهُ ذَاتَ يَوْمٍ فَقَالَ مَا فَعَلَ ذَلِكَ الإِنْسَانُ قَالُوا مَاتَ يَا رَسُولَ اللَّهِ قَالَ أَفَلاَ آذَنْتُمُونِي فَقَالُوا إِنَّهُ كَانَ كَذَا وَكَذَا قِصَّتَهُ قَالَ فَحَقَرُوا شَأْنَهُ قَالَ فَدُلُّونِي عَلَى قَبْرِهِ فَأَتَى قَبْرَهُ فَصَلَّى عَلَيْهِ