Sefer HaChinuch ספר החינוך

The commandment that the second Pesach (Pesach Sheni)-offering be eaten with matsot and marror (bitter herbs): That anyone who is obligated in Pesach Sheni eat from the meat of the Pesach-offering with matsot and bitter herbs, as it is stated (Numbers 9:11), “They shall eat it with unleavened bread and bitter herbs.”

[Concerning] the roots of the commandment, I have written about all of the matter of Pesach in the Order of Bo el Pharaoh (Sefer HaChinukh 5). And there is no doubt that the entire matter of Pesach Sheni is all from the foundation of the first — the thing is well-known.

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 chassah, [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 is fitting for us to look for romaine lettuce. Its details are also found in Pesachim.

And [it] is practiced at the time of the [Temple] by males, but not by females. As just like the slaughtering of the Pesach Sheni sacrifice is not obligatory [for them], but optional — as we said in the previous commandment — so too is the eating of matsah and marror with it not obligatory for them. And this is also elucidated there in Tractate Pesachim 95a. And one who transgresses this and eats the Pesach Sheni sacrifice without matsah and marror has violated this positive commandment.