Hadith Q81
[وَ] أَنَّ الِاسْتِطَابَةَ بِدُونِ ثَلَاثَةِ أَحْجَارٍ لَا يَكْفِي دُونَ الِاسْتِنْجَاءِ بِالْمَاءِ؛ لِأَنَّ الْمُسْتَطِيبَ بِدُونِ ثَلَاثَةِ أَحْجَارٍ عَاصٍ فِي فِعْلِهِ وَإِنِ اسْتَنْجَى بَعْدَهُ بِالْمَاءِ. وَالنَّهْيِ عَنِ الِاسْتِنْجَاءِ بِالْعِظَامِ وَالرَّجِيعِ
And performing istinja with less than three stones without using water is not sufficient. Because the one who performs istinja with less than three stones is sinful due to his action, even if he later performs istinja with water. Also, performing istinja with bones and dung is prohibited.
Hadith 81
Sayyiduna Salman (may Allah be pleased with him) narrates that the polytheists said, "Your companion has taught you everything, even it is possible that he has taught you the etiquettes of relieving yourselves." Sayyiduna Salman (may Allah be pleased with him) replied, "Yes, indeed, the Messenger of Allah (peace and blessings be upon him) has forbidden us from facing the Qiblah while relieving ourselves, or from cleaning ourselves with the right hand, or from cleaning ourselves with a bone or dung." And he said, "None of you should consider less than three stones to be sufficient."