3,644. When Challah Should Be Taken

Hilchos Bikkurim 8:2

When a person adds water to the flour and mixes it, he puts part aside as challah from the first part that is kneaded. This is derived from Numbers 15:20: “From the first of your dough.” This is the case so long as an omer of flour not mixed with water doesn’t remain in the mixing bowl. Let’s say that someone makes a condition that what he takes is challah for the dough, the leavening and the flour that remains. When it all becomes dough, what he put aside will be sanctified as challah. This is a permitted course of action.

Hilchos Bikkurim 8:3

If someone left the dough alone until it was all kneaded and only then took challah, it’s not a problem. If he doesn’t separate challah while it’s still dough, but rather he bakes the whole thing, he must take challah from the loaf, as has already been discussed.