He didn't say that. That verse is known as Pericope Adulterae. It does not exist in odler verisons of John. It's a latter-day addition. Read up on it. You are quoting something Jesus never said.
The Pericope Adulterae was not just that one verse but the passage of John 7:53-8:11. A quick check on Wiki shows that there IS evidence that the story was contained in early manuscripts. Per Wikipedia:
The pericope is not found in its canonical place in any of the earliest surviving Greek Gospel manuscripts; neither in the two 3rd century papyrus witnesses to John - P66 and P75; nor in the 4th century Codex Sinaiticus and Vaticanus, although all four of these manuscripts may acknowledge the existence of the passage via diacritical marks at the spot. The first surviving Greek manuscript to contain the pericope is the Latin/Greek diglot Codex Bezae of the late 4th or early 5th century. Papias (circa AD 125) refers to a story of Jesus and a woman "accused of many sins" as being found in the Gospel of the Hebrews, which may well refer to this passage; there is a very certain quotation of the pericope adulterae in the 3rd Century Syriac Didascalia Apostolorum; though without indicating John's Gospel.
I know you don't agree, but I believe the Bible we have today is what God has preserved for the world. It is exactly what he intended for us to have.