Leviticus 1:15

1:15 The priest must present it at the altar, pinch off its head and offer the head up in smoke on the altar, and its blood must be drained out against the side of the altar.

Leviticus 5:8

5:8 He must bring them to the priest and present first the one that is for a sin offering. The priest must pinch its head at the nape of its neck, but must not sever the head from the body.