It was narrated from Anas bin Malik that the Messenger of Allah ﷺ marked the grave of ‘Uthman bin Maz’un with a rock.  

عَنْ زَيْنَبَ بِنْتِ نُبَيْطٍ عَنْ أَنَسِ بْنِ مَالِكٍ أَنَّ رَسُولَ اللَّهِ ﷺ أَعْلَمَ قَبْرَ عُثْمَانَ بْنِ مَظْعُونٍ بِصَخْرَةٍ