Near the end of Parshat Shlach (Bamidbar 15:17-21) we read:
God said to Moshe, saying, “Speak to B’nai Yisrael and say to them: When you come to the Land to which I bring you it shall be that when you will eat the bread of the Land, you shall set aside a truma (portion) for God. From the first portion of your doughs you shall set aside challah (a loaf) as truma, like the truma gift of the threshing-floor, so shall you set it aside. From the first of your doughs shall you give to God a truma-offering, throughout your generations.”
For every batch of dough made out of one of the five types of grains (wheat, barley, rye, spelt, oats) a small piece of dough (challah) must be given to the Kohen, in the same way that they must receive a part of the produce of the field. Without separating the challah, the bread may not be eaten.
The minimum amount of dough from which challah must be taken is equivalent to the daily volume of the manna (the volume of 43.2 eggs) that B’nai Yisrael received in the desert.
Why are we taught about the mitzvah of taking challah right after the sin of the scouts?
According to Sforno, after the sin of the scouts, the mitzvah of challah became a necessity in order for the homes of the Israelites to enjoy God’s blessings. Yechezkel 44:30 states: “You are to give from the kneading bowl to the Kohen so that a blessing will rest on your house.” We also saw this with Eliyahu HaNavi (Melachim I 17:13-14) when he assisted the impoverished widow and commanded her to give him a small cake of the little flour that she had left. God said that in such a case the jar of flour in her house would not become empty during the remainder of the famine.
Our Shabbat bread is named after the challa that was separated out for the Kohen.
The Rama comments on the Shulcha Aruch, Orach Chayim 242:1:
We customarily knead a quantity of dough that is sufficient to become obligated in the mitzvah of challah, in the home. With this dough we bake breads that we will then break on Shabbat and holidays and one must not deviate from this custom.
Today, we don’t give the separated piece of dough to the Kohen. Rather, we either burn it or throw it in the garbage (wrapped in plastic). The amount of flour needed to make the dough in order to take challah and recite a bracha is more than 7 cups. If you are only using 5-7 cups of flour then you take challah without reciting a bracha.
Although taking challah is considered one of the three women’s mitzvot (along with family purity and lighting Shabbat candles) and a time when women specifically pray for whatever they need, if a man is baking bread then he is obligated to observe the mitzvah as well.
What is interesting about the mitzvah of challah is that it is one of the Mitzvot HaTluyot Ba’Aretz, a mitzvah that Biblically is specifically only observed in the Land of Israel yet it was taken on as a rabbinic mitzvah by those living abroad so that people in the Diaspora would not forget how to observe the mitzvah. Today, challah is considered a rabbinic mitzvah in Israel as we see in the Talmud, Ketubot 25a since all of the Jews in the world are not yet living in the Land of Israel. When the demographics change, it will become a Biblical mitzvah in Israel once again.