פירוש על מועד קטן 7:29

We learn (Yuma 81b) from verses that one must make a Tosefes (addition) to Shabbos, both when it enters and leaves, to add from Chol to Kodesh! Answer: That small amount is not important, since it is Kalshehu (any amount suffices); R. Gamliel would agree that one must add that Shi'ur [to Shevi'is, when it enters and leaves].

Explanation: Surely, mid'Oraisa it is Batel, but not like we said above; Rather, he (R. Gamliel) holds like R. Yishmael. R. Gamliel was meticulous about why the traditions about Tosefes Shevi'is and Nisuch ha'Mayim were taught together; Everywhere, these three Halachos (including also Aravah) are taught together. They were given [to Moshe] this way. Surely, Hash-m did so to teach [that all of them apply only when the Mikdash stands].

Explanation: We discuss a case in which we know that the [rain]water will not cease, and it is forbidden only due to the reason given (a decree lest he take from a low well); However, if they cease, there is an Isur like below. Regarding anything that ceases, we forbid lest he bring from a far place. Since he began to water it, [he must continue,] lest his field be ruined. Therefore, we establish here to discuss when it does not cease.