You cannot prove that, you just assume that, and even if He was, that does not mean He was referring to His body.
The physical body is not who Jesus was, it was just a temporary vehicle by which His soul could express itself while He was living on Earth. Jesus minimized the importance of the flesh and said that it is the spirit that quickens, but Christians go against what Jesus said and glorify His flesh. I find that rather ironic. Being born again has nothing to do with rising from the grave, it is related to spiritual rebirth.
John 3:5-7 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. Marvel not that I said unto thee, Ye must be born again.
John 6:63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.