Responsa for Shabbat 138:10

היודע שהוא שבת:

he may do work the whole of that day.<span class="x" onmousemove="('comment',' Viz., on the seventh after he set out, without any restrictions, since he certainly did not commence his journey on the Sabbath. ');">¹⁷</span> But that is obvious? — You might say, Since he did not set out on the Sabbath, he did not set out on the eve of the Sabbath either;<span class="x" onmousemove="('comment',' As it is unusual. ');">¹⁸</span> hence this man, even if he set out on Thursday. it shall be permitted him to do work on two days. Hence he informs us that sometimes one may come across a company and chance to set out [on a Friday]. HE WHO KNOWS THE ESSENTIAL LAW OF THE SABBATH. How do we know it? — Said R. Nahman in the name of Rabbah b. Abbuha, Two texts are written: Wherefore the children of Israel shall keep the Sabbath;<span class="x" onmousemove="('comment',' Ex. XXXI, 16. ');">¹⁹</span> and it is written, and ye shall keep my Sabbaths.<span class="x" onmousemove="('comment',' Lev. XIX, 3. ');">²⁰</span> How is this to be explained?<span class="x" onmousemove="('comment',' Sc. the employment of the sing. in one verse and the plural in the other. ');">²¹</span> 'Wherefore the children of Israel shall keep the Sabbath' [implies] one observance for many Sabbaths;<span class="x" onmousemove="('comment',' In the sense that if one desecrates many Sabbaths he fails in a single observance and is liable to one sin-offering only. ');">²²</span> [whereas] 'and ye shall keep my Sabbaths' [implies] one observance for each separate Sabbath.<span class="x" onmousemove="('comment',' Viz., that the desecration of each Sabbath entails a separate sacrifice. It then rests with the Rabbis to decide where each shall apply. ');">²³</span> R. Nahman b. Isaac demurred: On the contrary, the logic is the reverse: Wherefore the children of Israel shall keep the Sabbath [implies] one observance for each separate Sabbath; [whereas] 'and ye shall keep my Sabbaths' [implies] one observance for many Sabbaths.<span class="x" onmousemove="('comment',' R. Nahman b. Isaac agrees that the distinctions of the Mishnah follow from these texts, but he reverses their significance. ');">²⁴</span> HE WHO KNOWS THAT IT IS THE SABBATH.

Maharach Or Zarua Responsa