All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Crucifixion means to tie or nail someone alive to a large wooden beam and leave him until he dies. This form of killing is forbidden in Islam and it is not the punishment legislated for Hiraabah, although some scholars stated that it is the right punishment for Hiraabah as quoted by Shaykh Ibn Taymiyyah . However, it is forbidden according to the view of the majority of the jurists.
The method of crucifixion that is legislated for Hiraabah is that the person is killed with a sword first, and then he is crucified; this is the view of the majority of the scholars as stated by Shaykh Ibn Taymiyyah .
Other scholars also said that it is to tie the person alive first, and then kill him with spears while he is crucified. The Fiqh Encyclopedia reads:
The method of crucifying a mugger is as follows:
After careful and thorough examination of the statements of the jurists, it becomes clear that they agree that is not simply to crucify the mugger. Rather, he is tied to a wooden bar alive, and then he is left in this condition until he dies. However, they differed in opinion: Abu Haneefah, Maalik and Al-Awzaa'i said: He is crucified alive, and then he is killed with a spear. […] Ash-Shaafi’i and Ahmad said: “He is first killed and then crucified." [End of quote]
We could not find which of the two opinions Shaykh Ibn Taymiyyah adopted. All we found is that he reported both of them.
Allah knows best.