There's no mention of Matthias in the lists of disciples or followers of Jesus in Matthew, Mark, or Luke. According to Luke, writing in the first chapter of the Acts of the Apostles, Peter, in the days following the Ascension of Jesus, proposed to the assembled community of about one hundred and twenty people, that "one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us...one of these must become a witness with us to his resurrection..."
Two men were then suggested as candidates: "Joseph called Barsabbas who was also known as Justus, and Matthias. Then they prayed, 'Lord, you know everyone's heart. Show us which one of these two you have chosen to take the place in this ministry and apostleship from which Judas turned aside to go to his own place.'" When they cast lots, and the lot fell to Matthias, and he was added to the other eleven apostles.