I believe the earliest written Christian prohibition of abortion was in the Didacbe, 2nd century.
Many people believe that the Didache may have been written in the 40s or 50s A.D. by Paul and Barnabas.
“Thou shalt not slay the child by procuring abortion; nor, again, shalt thou destroy it after it is born” (Letter of Barnabas 19 from 74 AD).
“…And these were the accursed who conceived and caused abortion” (The Apocalypse of Peter 25, 137 AD).
“You shall not procure abortion, nor destroy a newborn child” (Didache 2:1 from 150 AD).