Where did God define which animals the Jews were allowed to eat as clean?
God explicitly defined the clean animals, fowl, and fish the Jews could eat in Leviticus chapter 11. That chapter set Israel's dietary distinctions between clean and unclean creatures. Peter, still under the law, later insisted he had never eaten anything common or unclean, showing those Old Testament food regulations were binding on him.
Les explains that Leviticus chapter 11 is where God had explicitly defined which types of animals, fowl, and fish the Jews could eat as clean. This did not include just the clean animals, but everything, the clean and the unclean. Later, in Peter's vision, Peter says he has never eaten anything common or unclean. Les points out that this is the crucial part many people miss: Peter says this because he is still under the law. Peter, like the followers of Christ and His earthly ministry, was a Jew who had believed Jesus was indeed the Christ, had repented, and been baptized. But he had not been told to stop temple worship, nor that he was no longer under the law. He was a typical Jewish believer who was not yet aware of the doctrines of grace, and so he was still much aware of the law. The voice then says, What God hath cleansed, call that not common.