הלכה על פסחים 79:12

From the laws of the commandment - that which they, may their memory be blessed, said (Pesachim 40a) that matsot require great watching, that they not come to rising, to the point that they, may their memory be blessed, obligated us to be careful that no water come upon [the wheat], even from the time of reaping, lest it come to rising (Mishneh Torah, Laws of Leavened and Unleavened Bread 5:9); and the rest of its details - are [all] elucidated in [the] first [section of] Pesach[im]. And the general rule is that there be a great guarding upon them that they not come to rising. And so [too,] what they explained about marror, that any bitter (mar) herb is included in the marror mentioned by the verse, and that a person fulfills his obligation on Pesach with any of them. As the commandment to us about marror is in memory of "they made life bitter for them" (Exodus 1:14), and the matter is remembered with any bitter herb. But nonetheless, the Sages, may their memory be blessed, chose for us (Pesachim 39a) to eat romaine lettuce. As with it, there is memory of the bitterness through the stalk, the taste of which is a little bitter, and there is also more beauty to the commandment than with other bitter herbs. And also its name is beautiful, as it is called chassa, [such that] a blessing is hinted in it - that the Merciful One pitied (chass) us and redeemed us from the hand of the harsh Egyptians. And from all of these [angles] there is arousal and remembrance in the heart of people of the matter of the miracles that were done for us in Egypt. And therefore, the law was fixed that it it fitting for us to look for romaine lettuce. Its details are also found in Pesachim.