daf bit: Menachot 41

[cellio]

The torah commands men to place tzitzit (fringes) on the corners of their garments. Is this an obligation on the person or on the garment? That is, are you required to wear a garment with corners (and attach tzitzit to them), or are you allowed to wear anything, but if it is a garment with corners, you must put tzitzit on it? In the g'mara, Rabbah bar R' Huna says it is an obligation on the garment, and not just the garment you're currently wearing -- all of your four-cornered garments must have tzitzit. There is some disagreement, and then the g'mara tells the following story:

An angel once found R' Kattina wearing a wrap (a garment that does not require tzitzit) and exclaimed: "Kattina, Kattina -- a wrap in summer and a cloak in winter, and what is to happen to the law of tzitzit?" R' Kattina answered: "and do you punish a person who omits to perform a positive precept?" "In a time of wrath, yes", the angel said. Now if you hold that tzitzit is an obligation on the person this makes sense, but if it's an obligation on the garment, why is there any punishment incurred? R' Kattina argued thus to the angel, asking "would the All-Merciful punish somebody for wearing a garment not subject to tzitzit?" The angel replied, "you find every excuse to free yourself from the law of tzitzit." (41a)

Despite the angel's opinion, as I understand it the halacha is according to Rabbah -- the obligation is on the garment, not on the person -- but there is really strong urging to seek out the opportunity because it's an established custom and not difficult.