TT #38 · Parashas Noach · Third Aliyah · Passuk 8:20
Then Noach built an altar to Hashem and, taking of every pure animal and of every pure bird, he offered burnt offerings on the altar.
Torah Temimah points to Zevachim 108b (shechutei chutz require an altar top).
E-Daf image — Zevachim 108b (size=3)
Shas / HebrewBooks PDF of the daf
English: “And Noach built an altar” — One is not liable for offerings slaughtered outside until they are brought up onto the top of an altar, as it is written “and Noach built an altar… and offered burnt offerings on the altar.”
Custom (halakhic derivation for the parameters of shechutei chutz).
Liability for offerings slaughtered outside requires that they be brought onto the top of an altar.