Aha, I found it: "Gif man husl aspiwe for druncennesse oððe for oferfýlle: fæste xl daga. Gif hund þæne spiwðan frete: fæste xl daga." That is- "If one vomits the host due to drunkenness or gluttony: he shall fast 40 days. If a dog then eats the vomit: he [the man] shall fast 40 days." This is the Old English Penitential, Bodleian Laud Misc. 482, f.
18a-
b.