Parashas Vayeilech · Passuk 31:21
and the many evils and troubles befall them—then this poem shall confront them as a witness, since it will never be lost from the mouth of their offspring. For I know what plans they are devising even now, before I bring them into the land that I promised on oath.
Torah Temimah points to Shabbat 138b (שבת קל"ח ב').
English: With regard to a bridegroom’s canopy, which has no roof but is entirely sloped, it is permitted to spread it and it is permitted to dismantle it on Shabbat. Rav Sheshet, son of Rav Idi, said: We only said that it is permitted in a case where its roof is not a handbreadth wide; however, if its roof is a handbreadth wide, it is prohibited. Furthermore, even when its roof is not a handbreadth wide, we only said that it is permitted where there is not the width of a handbreadth within three handbreadths of its roof; however, if it expands to the width of a handbreadth within three handbreadths of its roof, it is prohibited. And we only stated that it is permitted where there is not in its incline the width of a handbreadth; however, if there is in its incline the width of a handbreadth, it is prohibited. This halakha is in accordance with the principle that the inclines of tents, even though they are not actual roofs, are considered like tents. And we only said that this canopy is permitted where no part of the canopy descends to a handbreadth below the bed; however, if part of the canopy descends to a handbreadth below the bed, it is prohibited, as the bed itself becomes a roof, and the curtain is considered a wall. And Rav Sheshet, son of Rav Idi, also said: Wearing this felt hat is permitted on Shabbat, even though it has a wide brim and is similar to a tent. The Gemara raises a difficulty: Wasn’t it stated elsewhere that it is prohibited to wear a felt hat on Shabbat? The Gemara answers: This is not difficult: This latter statement, which prohibited wearing the hat, is referring to a case where its brim has the width of a handbreadth wide and is similar to a tent. That statement by Rav Sheshet, which permits wearing the hat, is referring to a case where its brim does not have the width of a handbreadth. The Gemara asks: But if that is so, if one extended his cloak a handbreadth beyond his head, would you also say that he is liable for making a tent? Rather, this is not difficult. The reason the hat is prohibited is not due to making a tent, but due to concern that the wind might blow the cap off one’s head and he will come to carry it by hand. The conflicting statements can be resolved as follows: This statement of Rav Sheshet, which permits wearing the hat, is referring to a case where it is fitted firmly on his head. That statement, which prohibits wearing the hat, is referring to a case where it is not fitted firmly. Rami bar Yeḥezkel sent to Rav Huna: Say to us, please, those excellent statements that you said to us in the name of Rav, two with regard to the halakhot of Shabbat, and one with regard to the Torah.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13
The Gemara uses this passuk inside a sugya (question, proof, or illustration). That is classical derash, but not labeled here as one of R' Yishmael’s 13 unless the move is explicit.
The Song lives ‘in the mouth of their seed’ without being forgotten: oral and written continuity are both part of how the witness survives every generation.