Where in the Bible does it state that Jesus told Peter that he would die a martyr's death?
Jesus told Peter that he would die a martyr’s death in John 21.
Truly, truly, I say to you, when you were younger, you used to gird yourself and walk wherever you wished; but when you grow old, you will stretch out your hands and someone else will gird you, and bring you where you do not wish to go. Now this He said, signifying by what kind of death he would glorify God. And when He had spoken this, He said to him, “Follow Me!” (NASB) John 21:18-19
Jesus predicted that Peter would die with his hands stretched out. What did Jesus mean? History provides the answer.
When Nero heard of those things, and observed that not only in Rome, but in every other place, a great multitude revolted daily from the worship of idols, and, condemning their old ways, went over to the new religion, he, an execrable and pernicious tyrant, sprung forward to raze the heavenly temple and destroy the true faith. He it was who first persecuted the servants of God; he crucified Peter, and slew Paul:
Lactantius, THE DIVINE INSTITUTES, chapter 2
Peter was crucified as a martyr. We believe that Peter died in A.D. 64. He was probably 60-70 years old when he died.
Peter served the Lord Jesus because he loved Him. That was Jesus’ question. Do you love Me? Peter, do you love Me? Do you love Jesus? If so, serve Him!