Parashas Vayikra · Aliyah 2 · Passuk 17
Chabad.org — English translation + Rashi
Torah Temima points to Zevachim 65b (verified; splitting by hand, not with a tool).
English: “And he shall split it with its wings; he shall not sever” — splitting is only by hand, as it is said (Shoftim 14) “and he tore it as one tears a kid, and there was nothing in his hand.”
Custom (gezerah shavah / word sense of shasa).
The bird is split by hand alone; a knife or other implement is not used for this step.