Punishment under Islamic Law for Practising Sihr 

According to Imam Malik, a person who practises sihr (sahir), but does not have it done for himself is similar to the person Allah describes in the following verse: (...and they learned what hurt them, and did not profit them, knowing well that whoso buys it shall have no share in the world to come.) (2:102). According the verse, Imam Malik believed that the Punishment for practising sihr is the death sentence. Al-Muwatta (628) 

According to Ibn Qudama, the Punishment for practising sihr is the death sentence. This view was reported by 'Umar, 'Uthman Ibn 'Affan, Ibn 'Umar, Hafsa, Jundub Ibn 'Abdullah, Jundub Ibn Ka'b, Qays Ibn Sa'd, and 'Umar Ibn 'Abdul'aziz. It is also the view of Abu Hanifa and Malik. 

According to al-Qurtubi, scholars of jurisprudence disagreed as to the punishment of a Muslim who practises sihr and a zimmi who practises sihr. According to Malik, the Punishment for a Muslim person who uses sihr himself that contains words of disbelief is the death sentence. He must not be requested to repent, as his repentance would not be accepted. This is because his sihr is something which he keeps secret, just like a disbeliever or an adulterer. This is also because Allah ('Azza wa Jalla) defines sihr as disbelief: (they taught not any man, without they said, 'We are but a temptation; do not disbelieve.) (2:102). This is also the view of Ahmad Ibn Hanbal, Abu Thawr, Ishaq, Ash-shafi'i (Ash-shafi'i held that a sahir should not be sentenced to death for simply practicing sihr, but rather, he should be killed, under al-qasas Law, if he has used a sihr which has claimed a life (quoted by Ibn Al-Mudir and others) and Abu Hanifa (Tafsir Al-Qurtubi (2/48)) 

According to Ibn Al-Mundir, the punishment for a person who admits to the use of sihr, containing words of disbelief is the death Punishment, if he does not repent, and also if the evidence against him shows that he has used words of disbelief. If he says that the sihr he has used did not contain words of disbelief, then he shall not be sentenced to death. If the sihr he used harmed a person so seriously as to deserve the death Punishment (under qasas law), then he must be sentenced to death, if he uses this sihr with intent to harm. However, if the sihr used does not merit the death penalty, then the penalty shall be blood money. Quoted from Tafsir Al-Qurtubi (2/48) 

Ibn Kathir argued that according to the verse: (...yet had they believed and been Allah-fearing, a recompense from Allah had been better, if they had but known) (2:103), a sahir does not commit such an act of disbelief when he uses sihr, a view which was held by Imam Ahmad Ibn Hanbal and a group of predecessors. However, the punishment for committing such act is the death penalty, according to a previous case reported by Ash-Shafi’I and Ahmad: "Sufyan Ibn 'Uyayna quoted 'Amru Ibn Dinar, 'I heard Bajala Ibn Abda say, "’Umar Ibn Al-Khattab instructed in writing to kill every sahir and sahira, so we killed three.'" 

According to Ibn Kathir, this hadith is transmitted by Al-Bukhari in his sahih (Fath Al-Bari: 6/257). Ibn Kathir stated: "The death penalty has another antecedent in the story of Hafsa, Umm Al-Muminin. One of her female servants performed sihr to her, so she ordered that the servant be killed, and the killing was carried out. 

According to Imam Ahmad, three companions of the Prophet (Peace and Blessing upon Him) confirmed that the punishment for a sahiris the death penalty. Tafsir Ibn Kathir : 1/144  

Al-Hafiz Ibn Hajar reported that according to Imam Malik, the punishment for practising sihr is similar to that applicable to az-zindiq (a person who does not believe in the Hereafter or who openly declares his faith, but conceals his kufr). His repentance should not be accepted, and he must be killed if it is confirmed that he is really a sahir. This is also the view of Imam Ahmad. 

According to Ash-Shafi'i, a sahir should be killed unless the sihr he has used had actually killed a person, in which case his killing must be counted as qisas. Fath Al-Bari: 10/236

Summary

According to the aforementioned views, it appears that the majority of scholars are of the opinion that a sahir should be killed, with the exception of Ash-Shafi'i, who said that a sahir should not be killed unless his sihr has killed a person, in which case his killing should be counted as qisas.