Where does it say in the bible that no man is free of sin as long as he is in the flesh or body?
I know of no scripture that says this directly. In fact, I do know of scriptures that contradict this idea. One group of people, called the Gnostics, taught that the flesh or body was totally sinful and that no good could come from the flesh. Part of this doctrine was that the spirit and the flesh were totally separate, since the spirit can be good, and that it didn’t matter what you did in the body because that would not affect your soul. This led some to sin without control and others to isolate themselves so they would not be tempted. Another result of this doctrine was that some taught that Jesus could not have actually been in flesh, since that would mean he became sinful. Most of the writings of the apostle John were designed to contradict this doctrine.
“For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.” (Hebrews 4:15) Since Jesus was without sin, either he did not come in the flesh or it is possible to be free of sin in the flesh.
“And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth.” (John 1:14)
“Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.” (1 John 4:2-3) Since Jesus came in the flesh (for to say otherwise is contradictory to God), then it is possible to be free of sin in the body.
That doesn’t mean that just anyone will succeed in being sinless in the flesh. If anyone but Jesus were able to do so then Jesus did not need to die on the cross. But that doesn’t mean that we can use the flesh to excuse sin.
Another side of the argument is that we can be free of sin in the body, because of God’s grace. When one is immersed (baptized) he washes away his sin (Acts 22:16). As he continues to walk in God’s light, the blood of Jesus continues to cleanse him of sin (1 John 1:7). So if we are in Christ, we are free from sin even while in the body.