Matthew 26:31-35 ESV -  Then Jesus said to them, "You will all fall away because of me this night. For it is written, 'I will strike the shepherd, and the sheep of the flock will be scattered.'  But after I am raised up, I will go before you to Galilee."  Peter answered him, "Though they all fall away because of you, I will never fall away."  Jesus said to him, "Truly, I tell you, this very night, before the rooster crows, you will deny me three times."  Peter said to him, "Even if I must die with you, I will not deny you!" And all the disciples said the same.Luke 22:54-62 ESV -  Then they seized him and led him away, bringing him into the high priest's house, and Peter was following at a distance.  And when they had kindled a fire in the middle of the courtyard and sat down together, Peter sat down among them.  Then a servant girl, seeing him as he sat in the light and looking closely at him, said, "This man also was with him."  But he denied it, saying, "Woman, I do not know him."  And a little later someone else saw him and said, "You also are one of them." But Peter said, "Man, I am not."  And after an interval of about an hour still another insisted, saying, "Certainly this man also was with him, for he too is a Galilean."  But Peter said, "Man, I do not know what you are talking about." And immediately, while he was still speaking, the rooster crowed.  And the Lord turned and looked at Peter. And Peter remembered the saying of the Lord, how he had said to him, "Before the rooster crows today, you will deny me three times."  And he went out and wept bitterly.
John 21:17-19 ESV -  He said to him the third time, "Simon, son of John, do you love me?" Peter was grieved because he said to him the third time, "Do you love me?" and he said to him, "Lord, you know everything; you know that I love you." Jesus said to him, "Feed my sheep.  Truly, truly, I say to you, when you were young, you used to dress yourself and walk wherever you wanted, but when you are old, you will stretch out your hands, and another will dress you and carry you where you do not want to go."  (This he said to show by what kind of death he was to glorify God.) And after saying this he said to him, "Follow me."
Among many other saints, the blessed apostle Peter was condemned to death, and crucified, as some do write, at Rome; albeit some others, and not without cause, do doubt thereof. Hegesippus saith that Nero sought matter against Peter to put him to death; which, when the people perceived, they entreated Peter with much ado that he would fly the city. Peter, through their importunity at length persuaded, prepared himself to avoid. But, coming to the gate, he saw the Lord Christ come to meet him, to whom he, worshipping, said, "Lord, whither dost Thou go?" To whom He answered and said, "I am come again to be crucified." By this, Peter, perceiving his suffering to be understood, returned into the city. Jerome saith that he was crucified, his head being down and his feet upward, himself so requiring, because he was (he said) unworthy to be crucified after the same form and manner as the Lord was. (http://www.ccel.org/f/foxe/martyrs/fox101.htm)
Titus 1:15-16 NASB -  To the pure, all things are pure; but to those who are defiled and unbelieving, nothing is pure, but both their mind and their conscience are defiled.  They profess to know God, but by their deeds they deny Him, being detestable and disobedient and worthless for any good deed.