ציטירנדיקער פּירוש אויף דברים 6:9

מפסיקין לק"ש – he took another matter, and this is how it should be read: colleagues engaged in the study of Torah interrupt their Torah study for the recitation of the Shema for its time is fixed, as it is written (Deuteronomy 6:9): “when you lie down and when you rise up.”

All of these [statements about] a house are written [in the Bible] - "the doorposts (mezuzot) of your house" (Deut. 6:9); "when you build a new house" (Deut. 22:8); "the one who owns the house shall come" (Lev. 14:35); "[a man] who sold a dwelling-house [in a walled city]" (Lev. 25:29); "[a man] who built a new house" (Deut. 20:5).

יחזור למקום שטעה – If between each section [of the Shema] he erred and did not know on which section he had interrupted (the appropriate order), and to which paragraph’s beginning he should return to, he should go back to the first interval – which is “והיה אם שמוע” (Deuteronomy 11:13-21). And Maimonides states that this is the first sentence: “ואהבת את ה' אלהיך” (Deuteronomy 6:5), and if he is in the middle of a section, he should stop [assuming that] he knows which section (of the Shema) that he is in, but if he doesn’t know which place and from which section he is in, he should stop and return to the beginning of that section. If he was reciting: “וכתבתם [על מזוזות]” and did not know if it was referring did not know if it was referring to “and you shall inscribe them” from the first section of the Shema (Deuteronomy 6:9) or the second section of the Shema (Deuteronomy 11:20), he should go back to the first mention of “you shall inscribe them” (Deuteronomy 6:9). But if he became doubtful after he had begun (the concluding verse of the second section of the Shema), “to the end that you and your children may endure” (Deuteronomy 11:21), he should not go back, since he is following the natural flow of the language [of the Shema].