Hadith: The Prophet (may Allah's peace and blessings be upon him) forbade making Istinjā’ with dung or bones, and said: They do not purify

عن أبي هريرة رضي الله عنه نهى النبي صلى الله عليه وسلم أَنْ يُسْتَنْجَى بِرَوْثٍ أو عَظْمٍ، وقال: «إِنَّهُمَا لَا تُطَهِّرَان».
[صحيح] - [رواه الدارقطني]

Abu Hurayrah (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) forbade making Istinjā’ with dung or bones, and said: "They do not purify."
Sahih/Authentic. - [Ad-Daaraqutni]

Explanation

Abu Hurayrah (may Allah be pleased with him) mentions that the Prophet (may Allah's peace and blessings be upon him) forbade the use of two things while making Istinjā’: dung and bones. Dung is forbidden because of its impurity; or in order to leave it so that the beasts of the Jinn may make use of it based upon his statement, may Allah's peace and blessings be upon him, that was narrated by At-Tirmidhi: "Do not make Istinjā’ with dung or bones, because it is the provision of your brothers from among the Jinn." As for bones, they are forbidden because of their smoothness, which makes them unable to remove impurity. It was also said that one can suck or chew bones in case of need. Another opinion justifies the forbiddance based upon the statement of the Prophet, may Allah's peace and blessings be upon him: "Bones are the provision of your brothers from among the Jinn," i.e. they find meat on bones more than the meat that was originally on them before having been eaten by humans. It was also said that the reason is that bones may cause injury. Then the Hadīth concludes by mentioning the reason behind the prohibition of using dung and bones in Istinjā’, stating that using them does not result in purification, hence, he said: "They do not purify.”