Does anyone know the first mention of abortion or birth control being a sin
There is, of course, the 5th Commandment.
However, abortion is specifically mentioned in the Didache (The Lord’s Teaching Through the Twelve Apostles to the Nations.) likely written between 50 AD to 100 AD. It could be considered the first catechism and the Didache is referred to by the Catholic Church in the footnotes of the CCC.
The complete Didache in this link:
earlychristianwritings.com/text/didache-roberts.html
Below the part specifically referring to abortion.
Chapter 2. The Second Commandment: Grave Sin Forbidden. And the second commandment of the Teaching; You shall not commit murder, you shall not commit adultery, you shall not commit pederasty, you shall not commit fornication, you shall not steal, you shall not practice magic, you shall not practice witchcraft,** you shall not murder a child by abortion nor kill that which is born.** You shall not covet the things of your neighbor, you shall not swear, you shall not bear false witness, you shall not speak evil, you shall bear no grudge. You shall not be double-minded nor double-tongued, for to be double-tongued is a snare of death. Your speech shall not be false, nor empty, but fulfilled by deed. You shall not be covetous, nor rapacious, nor a hypocrite, nor evil disposed, nor haughty. You shall not take evil counsel against your neighbor. You shall not hate any man; but some you shall reprove, and concerning some you shall pray, and some you shall love more than your own life.