It was customary during the festival for the governor to release to the crowd one prisoner, whomever they might choose.
References for Matthew 27:15
At that time there was a well-known prisoner named Jesus Barabbas.
When the crowd had come together, Pilate asked them, "Whom would you like me to release to you, Jesus Barabbas or Jesus who is called Christ?"
He knew that the leaders of the people had handed him over because of jealousy.
While he was serving as judge, his wife sent this message to him, "Leave that righteous man alone. I've suffered much today in a dream because of him."
But the chief priests and the elders persuaded the crowds to ask for Barabbas and kill Jesus.
The governor said, "Which of the two do you want me to release to you?" "Barabbas," they replied.
Pilate said, "Then what should I do with Jesus who is called Christ?" They all said, "Crucify him!"
But he said, "Why? What wrong has he done?" They shouted even louder, "Crucify him!"
Pilate saw that he was getting nowhere and that a riot was starting. So he took water and washed his hands in front of the crowd. "I'm innocent of this man's blood," he said. "It's your problem."
All the people replied, "Let his blood be on us and on our children."
Then he released Barabbas to them. He had Jesus whipped, then handed him over to be crucified.