This tradition has been transmitted by Hisham through a different chain of narrators to the same effect:
Rub it off (with a stone), and then scratch it (with finger) by pouring water, then sprinkle water upon it.

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا حَمَّادٌ، ح وَحَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا عِيسَى بْنُ يُونُسَ، ح وَحَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا حَمَّادٌ، - يَعْنِي ابْنَ سَلَمَةَ - عَنْ هِشَامٍ، بِهَذَا الْمَعْنَى قَالَ " حُتِّيهِ ثُمَّ اقْرُصِيهِ بِالْمَاءِ ثُمَّ انْضَحِيهِ " .