I heard Ash-Shuʿbi saying, "I was told by a man who had passed with the Prophet ﷺ by a grave that was separate from the other graves that he (the Prophet ﷺ ) led them in the prayer and they prayed behind him." I said, "O Abu ʿAmr! Who narrated that to you?" He replied, "Ibn ʿAbbas."  

أَخْبَرَنِي مَنْ مَرَّ مَعَ النَّبِيِّ ﷺ عَلَى قَبْرٍ مَنْبُوذٍ فَأَمَّهُمْ وَصَلَّوْا خَلْفَهُ قُلْتُ مَنْ حَدَّثَكَ هَذَا يَا أَبَا عَمْرٍو قَالَ 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