The apostles were at the Mount of Olives when this happened, so they walked the half mile back to Jerusalem.
Then they went to the upstairs room of the house where they were staying. Here is the list of those who were present: Peter, John, James, Andrew, Philip, Thomas, Bartholomew, Matthew, James (son of Alphaeus), Simon (the Zealot), and Judas (son of James).
They all met together continually for prayer, along with Mary the mother of Jesus, several other women, and the brothers of Jesus.
During this time, on a day when about 120 believers were present, Peter stood up and addressed them as follows:
"Brothers, it was necessary for the Scriptures to be fulfilled concerning Judas, who guided the Temple police to arrest Jesus. This was predicted long ago by the Holy Spirit, speaking through King David.
Judas was one of us, chosen to share in the ministry with us."
(Judas bought a field with the money he received for his treachery, and falling there, he burst open, spilling out his intestines.
The news of his death spread rapidly among all the people of Jerusalem, and they gave the place the Aramaic name Akeldama, which means "Field of Blood.")
Peter continued, "This was predicted in the book of Psalms, where it says, 'Let his home become desolate, with no one living in it.' And again, 'Let his position be given to someone else.' "
"So now we must choose another man to take Judas's place. It must be someone who has been with us all the time that we were with the Lord Jesus --
from the time he was baptized by John until the day he was taken from us into heaven. Whoever is chosen will join us as a witness of Jesus' resurrection."
So they nominated two men: Joseph called Barsabbas (also known as Justus) and Matthias.
Then they all prayed for the right man to be chosen. "O Lord," they said, "you know every heart. Show us which of these men you have chosen
as an apostle to replace Judas the traitor in this ministry, for he has deserted us and gone where he belongs."
Then they cast lots, and in this way Matthias was chosen and became an apostle with the other eleven.