[If] one takes sourdough from a wheat dough and places [it] into a rice dough, if it has the taste of grain, it is obligated in challah; but if not, it is exempt. If so, regarding what did they say [that] tevel forbids [a mixture even] in minute concentration? A kind with its [like] kind; but with an unlike kind, [tevel prohibits only] when [it] imparts taste [to the mixture].