Matthew 16:7

‘And they reasoned among themselves, saying, “We took no bread.” '

But they misunderstood His words and took them literally. They thought that He also was talking about their having no bread, and so vigorous discussions took place about what they were going to do in order to remedy the situation.

Leaven was the old dough kept back from a previous baking which when put in with the new flour mix ‘leavened' the whole making it light and airy. Its swift and insidious action was well known. This should have warned them that He was speaking pictorially. For why otherwise should he have spoken of the leaven and not the bread itself? It was bread that they were lacking. Alternatively they might have taken His words as a shorthand expression for leavened bread.