Midrasch zu Bamidbar 29:12

(Bamidbar 28:24) "As these shall you offer each day for seven days.": Why is this written? For (were it not written) it would follow, (otherwise), viz.: Since Succoth requires a festival offering, and Pesach requires a festival offering, therefore, just as the offering of Succoth decreases progressively (each day [viz. Bamidbar 29:12-13]) so should that of Pesach decrease progressively. It is, therefore, written "As these shall you offer each day" — neither to increase or to decrease. "in addition to the continual burnt-offering shall it be offered, and its libation": We are hereby apprised that the mussafim (of Pesach) are sacrificed only between the two (temidim).

(Bamidbar 29:12) "And on the fifteenth day of the seventh month, a calling of holiness shall there be for you": Scripture makes it mandatory. (Ibid. 13) "And you shall present a burnt-offering, a fire-offering, a sweet savor to the L-rd": If one found bullocks but not rams, or rams but not lambs, I might think that he does not sacrifice any until he finds all. It is, therefore, written (Vayikra 23:36) "Seven days shall you present a fire-offering to the L-rd," implying (if he found) even one. I might think that even if all are found, (he may present only one). It is, therefore, written "thirteen young bullocks, two rams, etc." Whence is derived the water libation on Succoth? R. Akiva says: It is written that the omer is to be brought on Pesach so that the grain be blessed for you, and bring bikkurim on Shavuoth so that the fruits be blessed for you. Also, (by induction), present a water libation on Succoth so that the rains of the year be blessed for you. R. Yehudah says: On the second day it is written "veniskehem" ("and their libations"); on the sixth, "unesachehah"; and on the seventh, "kemishpatam" — (superfluous) Mem Yod Mem, which spells "mayim" (water) — whence the water libation (on Succoth) is Scripturally intimated. R. Nathan says: What is the intent of (Bamidbar 28:7) "On the holy place (the altar) pour a pouring"? To include the water (libation).