Bemidbar

When we are commanded about wearing tzitzis, the Torah includes two mitzvos. In addition to the mitzvah of wearing tzitzis threads on the corners of the garment, there is an additional mitzvah that some of the tzitzis threads should be dyed with a special dye called techeiles. (There is a dispute among the Rishonim how many of the tzitzis threads are to be dyed techeiles.) This dye must be made from a species called chilazon (Tosefta Menachos 9:6).