In the book of Numbers, the Lord speaks to the Israelites through his servant Moses and commands them to wear tassels (or tsitsit) on the corners of their garments. The tassels must include a blue thread.
The Biblical passage reads as follows:
“The Lord spoke to Moses, saying: Speak to the Israelite people and instruct them to make for themselves tassels (tsitsit) on the corners of their garments throughout the generations; let them attach a cord of blue to the tassel at each corner. That shall be your tassel; look at it [the blue cord] and recall all the commandments of the Lord and observe them, so that you do not follow your heart and your eyes in your lustful urge. Thus you shall be reminded to observe all my commandments and to be holy to your God. I am the Lord your God, who brought you out of the land of Egypt to be your God: I the Lord your God.”
The tassels were in fact extensions of the hem, as we learn from innumerable illustrations in ancient Near Eastern art.