It is not permissible to slaughter
camels, cows, sheep, or anything else over the graves. This is an act of major Shirk (associating others with Allah in His Divinity or worship) that puts the doer beyond the pale of Islam; if by doing so they mean to draw close to those buried in the graves, hoping for their blessing. It is only Allah Who can be drawn close to in this way. Allah (Exalted be He) says (what means):
Say (O Muhammad صلى الله عليه وسلم): "Verily, my Salât (prayer), my sacrifice, my living, and my dying are for Allâh, the Lord of the ‘Alamîn (mankind, jinn and all that exists).
"He has no partner. And of this I have been commanded, and I am the first of the Muslims."
Treating sick people by reciting over them Qur’an, Adhkar (invocations and remembrances said at certain times on a regular basis) and Du‘a’ (supplications) that are authentically reported from the Prophet (peace be upon him) is lawful. However, going to the people you mentioned for them to recite Ayahs over the sick and then order the slaughtering of rams or bulls, for example, is not permissible. (Part No. 1; Page No. 200) In fact, this type of Ruqyah (saying supplications over the sick seeking healing) is Bid‘ah (innovation in religion) and involves cheating people out of their money. Finally, it is an act of Shirk if the mentioned animals are slaughtered as a sacrifice for the Jinn, the dead, or others, to ward off evil or bring a benefit.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.