The noble Prophet Muhammad (PBUH) said:
He is not one of us who observes bird omens or has that done for him, who tells fortunes or has his fortune told, who does witchcraft or has witchcraft done for him. Whoever goes to a fortune teller and believes what he says has disbelieved in that which was revealed to Muhammad Al-Bazaar with a Jayyid isnaad; Al-Tabrani
In Saheeh Al-Bukhari and Saheeh Muslim, it is narrated from Abu Huraira that the Messenger of Allah (PBUH) said:
Avoid the seven (sins) that doom a person to Hell
They said: "O Messenger of Allah, what are they?'
He (PBUH) said:
Associating others with Allah (Shirk), Witchcraft, killing a soul whom Allah has forbidden killing (except in cases required by Islamic law), consuming interest (Riba), consuming the orphan's wealth, running away from the battlefield, and slandering chaste and innocent believing women.
And he (PBUH) also said:
Whoever goes to a fortune teller and believes what he says has nothing to do with what Allah revealed to Muhammad (PBUH), and whoever goes to him and does not believe him, his prayers will not be accepted for forty days. Fath Al-Baari, 5/393; Muslim, 1/92
Witchcraft often produces the opposite effect to what was hoped for, so the one who asks for witchcraft to be done ends up being afflicted. Many women have performed witchcraft on their husbands to make them love them, and not take a second wife, then the husband divorces his wife as a result of witchcraft.
Allah (SWT) says:
But the evil plot encompasses only him who makes it Al-Qur’an, 35:43
Sheikh Hafiz Al-Hukami said: It is Haram to undo the spell of a witch by means of further witchcraft, because that is cooperating with the witch and approving of his action, and drawing close to the Shaytaan by various means in order to undo what was done to the victim.
Hence Al-Hasan said: No one regards witchcraft as permissible except the witch. Hence you see many evil witches during times when there is no sword to deter them, deliberately bewitching people, whether they love them or hate them, in order to force them to ask him to undo it, as a means of taking people's wealth unlawfully and misleading them.
Ma'aarij AUQubool, 1/530
The jinn gain a great deal of power over the magician during his lifetime. They make some magicians sick and kill others, then gain power over their offspring after they are gone, because they know that they are weak and powerless. That is because the devils only serve the witch and fulfil some of his requests because he has disbelieved in Allah (SWT), either in word, by reciting spells which involve associating others with Allah (Shirk) and venerating the jinn, or in deed, by showing disrespect to the Word of Allah (the Qur'an). Hence the jinn serve him and fulfil some of his requests, whilst the magician fulfils all the demands of the devil, because he is in a position of humiliation and has no choice.
Frequenting magicians is a great evil. The jinn dictate their demands and conditions to the witch who in turn transmits them to those who frequent him, such as slaughtering birds in a specified manner, eating certain food, or staying alone in a darkened room for a certain period of time. The more humiliated & sinful the witch becomes, the more pleased the devil becomes, and the witch gets some of his requests granted (at the cost of his soul). Allah (SWT) spoke the truth when He said:
And verily, there were men among mankind who took shelter with the males among the jinn, but they (jinn) increased them (mankind) in sin and transgression. Al-Qur’an, 72:6