Matthew 16:6

‘And Jesus said to them, “Take heed and beware of the leaven of the Pharisees and Sadducees.” '

Seeing their concern over mere bread (compare Matthew 4:4) Jesus then intervened with a comment which was designed to make them recognise that there was more to worry about than the lack of bread. Let them rather be concerned about the leaven of the Pharisees and Sadducees, and the threat that it posed. It was that that they should really be concerned about, the insidious teaching of the Pharisees and Sadducees which was undermining His ministry and bringing them all under threat, especially their joint teaching about the requirement for ‘signs' and the implication that He was not the Messiah.