Pratimokshasutra of the Sarvastivadins 1.8

1.8

「若比丘，共諸比丘入戒法中，不還戒、戒羸不出，行婬法乃至共畜生，是比丘得波羅夷罪，不應共事。

“Whatever bhikṣu, having entered into the discipline of the precepts together with the bhikṣus, without having returned the precepts and without having declared weakness of the precepts, engages in sexual intercourse, even down to an animal, this bhikṣu incurs a pārājika offense and is unfit to dwell in communion.