Siman 398

Whether there is a basis for mourning from the Torah is a dispute among the authorities. For the Rambam in the second chapter and the Geonim hold that mourning on the first day is from the Torah, as it is written: "And had I eaten the sin offering today, would it have been good in the eyes of Hashem?", and it is written: "And the end thereof as a bitter day." And the essence of bitterness is one day. And one can bring a proof for their words from the fact that it is forbidden to wear tefillin on the first day. And if it were rabbinic, how could they nullify him from the mitzvah of tefillin, which is a positive commandment from the Torah? And the Rashba and the Rosh wrote at the beginning of the third chapter that this is when the burial was on the day of death, it is from the Torah. But if the burial is on the next day, there is no mourning of the Torah here. (And according to this, the proof of tefillin is gone, for even in such a case it is forbidden to wear tefillin. Rather, one must say as will be explained according to the opinion of Rabbeinu Tam and the Ri. And the opinion of the Ra'avad is that even the decree of thirty days for pressing clothes and haircutting is from the Torah, for we learn it from a nazir, as the Rosh and the Tur wrote. And in this, we have not found him a companion.)

But the opinion of our teachers the Tosafists, and at their head our teacher Tam and the Ri, is that there is no basis at all for mourning from the Torah, only aninut is from the Torah regarding the eating of kodashim and ma'aser sheni. And the sages added further prohibitions to it, as is explained at the beginning of the third chapter of Berakhot. And aninut and mourning are two different things, and that which is forbidden in this is permitted in that, except for marital relations which is forbidden in both. And even in this there are those who disagree as I wrote in siman 341, and the essence of mourning is rabbinic. And the Rosh agreed with their words, and wrote that the reason for tefillin is that since tefillin are glory, and the mourner on the first day has defiled his horn in the dust – it is not proper that he place the glory in the place of the ashes. And through "sit and do not act" the sages nullified for him the mitzvah of tefillin on the first day, see there. And they wrote that even according to the opinion of the Geonim that it is from the Torah, it is only for the seven dead specified in Parshat Emor, and not for those they added as I wrote in siman 374. (And in Be'er HaGolah, laws of mourning, he wrote that thus the sages supported that the essence of mourning from the Torah is the first day etc. Thus far his words. And his intention is not an asmachta, but rather his intention is that the sages relied on this to rule so, see there. And be precise and you will find it easy.)