Where does it say that Judas stole from the money bag?

AnswerJohn 12:6 (KJV): "This he said, not that he cared for the poor; but because he was a thief, and had the bag, and bare what was put therein." ("He" in this sentence is referring to Judas; a woman had just broken an expensive jar of perfume over Jesus' feet, and Judas complained that it would have been better to sell the perfume and give the money to the poor.)

However, this was not written when it happened but many years later, and it's at least possible that John's opinion of Judas had been colored somewhat by subsequent events; there are no other scriptures mentioning that anyone was particularly concerned about Judas being the treasurer of the group at the time. Later, at the Last Supper, Jesus tells Judas to go and do something without specifically stating what it is; the apostles assume that Jesus is sending him out to buy something or possibly to give money to the poor based on the fact that Judas is still treasurer even then.