פירוש על ערכין 4:55

Everyone is obligated in tsitsit: if they have a cloak [that requires it]. But if one is not able to buy [such] a cloak, they are not obligated - not according to the opinion that says the obligation is on the cloak, nor according to the opinion that says the obligation is on the person (Menachot 41a). For the only difference between them is about clothing in the cupboard. And even though an angel said to Rav Ketina, "[If you wear] a linen cloak in the summer and a sarbela (a coat which has only two corners) in the winter, what will become of tsitsit." That is to say, "Are you trying to find ruses to become exempt from tsitsit," as is explained over there. And he said to him, "Do you punish us (about it) [for a positive commandment]?" He said [back] to him, "At a time of [Divine] anger, we do punish [for it]." [Nevertheless,] that is specifically in those times when all [standard] cloaks had four corners that there was a punishment for the one that altered it to become exempt. But today - since most cloaks are exempt, there is no punishment for one who does not buy a cloak of four corners.