Chabad.org — English translation + Rashi
Torah Temimah points to Menachos 63a.
E-Daf.com — Menachos 63a
HebrewBooks / Shas PDF
English: Rabbi Yehudah said: Soften [the rule] for one who says “I am obligated to an oven-baked minchah” so that he not bring half loaves and half wafers; the verse says “an offering of minchah” — one offering I said, and not two or three.
Miyut against mixing challos and rekikin in one vow — the vow is for one offering, not a combination of two types.
A single oven-minchah vow is fulfilled with either challos or rekikin, not a mixed half-and-half. Rabbi Yehudah reads “an offering of minchah” as requiring one coherent offering; the donor may not discharge the vow by bringing half loaves and half wafers.