Our fathers ate the manna in the wilderness; as it is written, ‘He gave them bread from heaven to eat.’”
John 6:31

They used the example of manna because Passover was at hand, and they were thinking of unleavened bread.

They weren't wrong to ask for some kind of divine confirmation of his claim, but the sign that Yeshua offered wasn't the kind they were expecting. This is the sign that Yeshua is the Messiah: His existence and everything about his life testified to his identity.