Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
57. It is a positive commandment to separate Challah (a small amount of dough) from a batch of dough and give it to a Cohain
As it states in Bamidbar (Numbers) 15:20, "The first of your dough you shall separate." Challah separation is required if the dough is one of the five grains and if the dough is the size of forty-three and one fifth eggs. According to the law of the Sages, outside of the Land of Israel, challah should be separated and burned, so a person does not forget the law of Challah. According to Torah law there is no minimum amount needed to be separated. Anyone that separates any amount from the dough has fulfilled his Torah obligation.