Reference for Pesachim 58:12

שלא במינן אטו מינן לא גזר שלא בזמנו בין במינן בין שלא במינן מותרין כר' שמעון

For Rab and Samuel both said: All forbidden things of the Torah, [if mixed] with their own kind, [render forbidden the mixture even] when there is a minute quantity; [if] with a different kind, [only] when [the forbidden element] imparts its taste. Now Rab forbade leaven in its time [when mixed] with a different kind on account of [a mixture with] its own kind.

Jastrow