It is found not only in first century AD New Testament sources like Matthew, Mark, Luke, John, Acts, Paul’s letters, Hebrews, and First Peter, but also in other ancient Christian sources not found in the Bible, like Ignatius, Polycarp, Epistle of Barnabas, Justin Martyr, Hegesippus, Mileto of Sardis, Tertullian, Irenaeus, Clement of Alexandria, Origin, and the Diatessaron. In fact, Jesus’ crucifixion is even attested by early non-Christian (even anti-Christian) sources like Josephus and Lucian, and in a round-about way by Tacitus.
The crucifixion of Jesus is an accepted fact in the scholarly world. In fact, almost the only ones who reject this fact are Muslims. According to the Qur'an, Jesus only appeared to be crucified. The Qur’an refers to Jews boasting about killing Christ:
“That they said (in boast), ‘We killed Christ Jesus The son of Mary, The Apostle of God’’—But they killed him not, Nor crucified him, But so it was made to appear to them (Sura 4:157).The Qur’an contains the words of Muhammad as remembered by his earliest followers. The sources I cited above which affirm the crucifixion of Jesus were written from 400 to almost 600 years before the time of Muhammad.
So why does Muhammad deny something as historically solid as the crucifixion of Jesus?
Muhammad was hoodwinked by a group of people we often call "Gnostics", who believed that Jesus was divine but not truly human. As a result, they didn’t believe Jesus could really die. So about 200 to 300 years after Jesus’ death, they argued that Jesus was not really crucified and that he died only in appearance. For example, one of the Gnostic writings, says,
“I did not succumb to them as they had planned. But I was not afflicted at all. Those who were there punished me. And I did not die in reality but in appearance…They struck me with the reed; it was another, Simon, who bore the cross on his shoulder. It was another upon whom they placed the crown of thorns. But I was rejoicing in the height over all the wealth of the archons…and I was laughing at their ignorance…for I was altering my shapes…” (Second treatise of the Great Seth 55-56).A similar document says, “The Savior said to me, ‘He whom you saw on the tree, glad and laughing, this is the living Jesus. But this one into whose hand and feet they drive the nails in his fleshly part, which is the substitute…” (Apocalypse of Peter 81)
So supposedly, while “the living Jesus” was being crucified—one of the most brutal tortures imaginable—he was laughing because the nails had been driven into the hands of a substitute?! This is the pool of ideas from which Muhammad got his idea that Jesus was never crucified.
While Muhammad accepted the Gnostic idea that Jesus was not crucified, he rejected their idea that Jesus was a divine being. That is interesting because their reason for thinking Jesus was not crucified in the first place was because, in their view, Jesus was divine, but not not really human!
So if you think it is historically plausible that Jesus was not human at all, and that he was some kind of shape-shifter ("I was altering my shapes"), and was laughing on the cross because it was not really him on the cross but a substitute, then maybe Muhammad’s view of Jesus not being crucified might make sense to you.
But if, like virtually all scholars, you dismiss such nonsense as unhistorical, then you should also recognize that Muhammad was not a prophet of God at all, but was merely an ordinary man who was duped into believing this, and the numerous other historical errors he relates in the Qur’an.