One of the three miracles that took place in Sarah's tent, (which returned when Rivka married Yitzchak) was that the candles burned for the whole week, the challah stayed fresh from week to week, and there was a cloud over the tent.
Rashi (Bereishit 24:67) does say regarding the Challah miracle 'a bracha was found in the dough.' ברכה מצויה בעיסה. but does not say what the miracle was, it could be anything, in fact some say that the miracle was that the dough increased miraculously, (Sapirstein chumash.)
Does anyone know the source for the Challah lasting all week in Sarah's tent?