Siman 603

The Tur brought a Yerushalmi: Rav Chiyya commanded Rav: If you are able to eat in purity the entire year, eat. And if not, eat for seven days in the year (Chapter 1 of Shabbat, Halacha 3). And the Raviah wrote: I have received that these are the seven days between Rosh Hashanah and Yom Kippur. And he calls them "seven days" because on Rosh Hashanah he would eat in purity anyway, as one is obligated to purify himself on the festival. Therefore, he wrote that even one who is not careful throughout the entire year regarding the bread of a non-Jew—during the Ten Days of Repentance it is proper to be careful. See there.

And it appears in my humble opinion that this is only regarding matters in which there is no prohibition according to the law, for in this it is only a mere enhancement, and it is proper to enhance this during these days. But matters regarding which there are poskim who rule to prohibit according to the law, but they conduct themselves according to the opinion of those who permit, such as chadash in the diaspora, or meat without a sircha, and the like—it is impossible to conduct oneself so during the Ten Days of Repentance. For since they do not eat [these] during these days—behold, it is as if they accepted upon themselves the opinions of those who prohibit. And if so, how shall they eat [them] afterwards? (See Magen Avraham that regarding the bread of a non-Jew—one should be careful at the very least regarding the throwing of a wood chip. And they should be careful not to take challah from the dough of a non-Jew. And what he wrote regarding one who is traveling on a road etc., see there—it requires further study. And a woman who found a dead child with her—she should accept repentance. And one who sent a messenger, and he was killed on the road, also—so, see there. And from the essence of the law there is room for doubt, and we have explained it in another place. And be precise and you will find it easy.)

Every person must examine their deeds during the ten days of repentance and repent for them. A doubtful transgression is more severe than a certain transgression and requires more effort for repentance, for one feels more regret when he knows he has committed a sin than when he does not know with certainty. Therefore, the asham talui sacrifice is more expensive than a certain asham, as is explained in the Torah, Mishnah, and Gemara. Furthermore, regarding a doubt, the yetzer hara incites him more than regarding a certainty, by saying that there is no transgression here. Therefore, one requires more strengthening for this. And one who comes to be purified is assisted. With the help of Heaven, the laws of Rosh Hashanah are concluded.