The Martyrdom of Saint Peter

Then both Peter and Paul were led away from the presence of Nero.

And Paul was beheaded on the Ostesian road.

And Peter, having come to the cross, said: Since my Lord Jesus Christ, who came down from the heaven upon the earth, was raised upon the cross upright, and he has graciously willed to call to heaven me, who am of the earth, my cross ought to be fixed head downmost, so as to direct my feet towards heaven; for I am not worthy to be crucified like my Lord.

Then, having reversed the cross, they nailed his feet up. And the multitude was assembled reviling Caesar, and wishing to kill him.

But Peter restrained them, saying: a few days ago, being exhorted by the brethren, I was going away; and my Lord Jesus Christ met me, and having adored him, I said, Lord, where are you going?
And he said to me, I am going to Rome to be crucified.
And I said to him, Lord, were you not crucified once for all? And the Lord answering, said, I saw you fleeing from death, and I wish to be crucified instead of you.
And I said, Lord, I go; I fulfil your command. And he said to me, Fear not, for am with you.
On this account, then, children, do not hinder my going; for already my feet are going on the road to heaven.
Do not grieve, therefore, but rather rejoice with me, for today I receive the fruit of my labours.

And thus: speaking, he said: I thank you, good Shepherd, that the sheep which you have entrusted to me, sympathize with me; I ask, then, that with me they may have a part in your kingdom. And having thus spoken, he gave up the ghost. And immediately there appeared men glorious and strange in appearance; and they said: We are here, on account of the holy and chief apostles, from Jerusalem. And they, along with Marcellus, an illustrious man, who, having left Simon, had believed in Peter, took up his body secretly, and put it under the terebinth near the place for the exhibition of sea-fights in the place called the Vatican.

And the men who had said that they came from Jerusalem said to the people: Rejoice, and be exceeding glad, because you have been deemed worthy to have great champions. And know that Nero himself, after these not many days, will be utterly destroyed, and his kingdom shall be given to another.

(Acts of Peter & Paul)