Midrash sa Levitico 2:4: Sifra

1) (Vayikra 2:4): "And if you bring (an offering of a meal-offering, etc."): "if you bring" — it is optional (not mandatory). "an offer (korban) of a meal-offering": R. Yehudah said: Whence is it derived that if one says: "I take upon myself a baked meal-offering" that he may not bring part cakes and part wafers? From "an offer" — he brings on offering, and not cakes and wafers.

1) (Vayikra 7:9) ("And every meal-offering that is baked in the oven, and all that is made in the stewing-pan and on the griddle, to the Cohein who offers it up, to him shall it be. (Vayikra 7:10) And every meal-offering mixed with oil or dry, to all the sons of Aaron shall it be, one man as well as another.") R. Yossi b. R. Yehudah said: Whence is it derived that if one said: "I vow (to bring) a burnt-offering of meal baked in an oven" that he should not bring half-cakes (challoth) and half-wafers (rekikim)? From "every meal-offering that is baked in the oven, and all that is made in the stewing-pan and on the griddle … and every meal-offering mixed with oil or dry." Just as the latter ("stewing-pan and griddle" and "mixed with oil or dry") are (respectively) two kinds, so, these ("cakes and wafers" — viz. Vayikra 2:4 ("baked in an oven…) cakes and wafers") are two kinds (and cannot combine with each other to constitute a vow connoting a single kind). "and all that is made in the marchesheth (stewing-pan) and the machavath (griddle)." These terms connote the vessels (themselves) and not their products. Just as a tanur ("oven") is a vessel, so marchesheth and machavath are vessels.

2) R. Shimon said: Is "offer" written twice? It is written only once, and this is followed by "cakes and wafers" — so that if he wishes to bring cakes he may do so; if he wishes to bring wafers, he may do so; if he wishes to bring cakes and wafers he may do so. And when he takes the fistful (of the cakes and wafers after baking), he mixes both of them (i.e., their pieces) together and takes the fistful. If in taking the fistful only one of them (i.e., one variety) comes up in his hand, it is sufficient.