מדרש על יבמות 127:7

R. Isaac said: "Our father Isaac was impotent, as it is said (Gen. 25, 21) And Isaac entreated the Lord (L'nochach) opposite his wife. It is not stated Al (concerning) his wife, but it is said L'nochach (opposite) his wife. Infer from this that both of them were impotent [and therefore both prayed."] If so, then why is it written: And the Lord was entreated of him? Entreated of them, it ought to be? This is because the prayer of a righteous (Isaac), the son of a righteous, is not like the prayer of a righteous (Rebecca), son (daughter) of a wicked. R. Isaac said: "Why were our ancestors childless [until they prayed?] Because the Holy One, praised be He! desires [to hear] the prayers of the righteous." R. Isaac said: "Why are the prayers of the righteous symbolized by a shovel? Because just as the shovel turns the grain at the threshing floor from place to place, so the prayers of the righteous turn the dispensations of the Lord from anger to mercy." R. Ami said: "Sarah our mother was a Tumtum, as it is said (Is. 51, 1) Look unto the rock, (Ib. b) whence ye were hewn, and to the hole of the pit whence they were dug out. And immediately follows: Look unto Abraham your father, and unto Sarah that bore you." 11. Nachman, in the name of Rabba b. Abahu, said: "Our mother Sarah was wombless, as it is said (Gen. 11, 30) But Sarah was barren, she had no child; i.e., even a place for a child (womb) she had not."