[If] one separates his challah [from] flour, it is not challah, and is [considered] stolen [as long as it remains] in the hands of the Kohen [to whom it was given]. The dough itself is obligated in challah; and the flour, if it consists of the [minimum] measure, is obligated in challah and is forbidden to non-Kohanim; [these are] the words of R' Yehoshua. They said to him: There was an incident [in which flour was designated as challah] and a sage who was a non-Kohen grabbed it. He said to them: He, too, ruined matters for himself, [though] he fixed them for others.