Sifrei Bamidbar 17

(Bamidbar 5:25) "And the Cohein shall take from the hand of the woman": and not from the hand of her representative — If she were in her menstrual period, she did not drink, (being forbidden to enter the azarah at that time). (5:24) "And he shall make the woman drink": Why is it written again (Ibid. 27) "and he shall make her drink the water"? For if the scroll were erased and she said "I will not drink," they shake her and make her drink perforce. These are the words of R. Akiva. R. Shimon says: (Ibid. 26) "and then he shall make the woman drink": What is the intent of this? It is written afterwards "and he shall make her drink the water"! (To indicate that) three things are categorically required for (the validity of) the sotah (procedure): the erasure of the scroll, the offering of the fistful, and her acceptance of the oath. If the scroll were erased and she said; I am tamei, the waters are spilled out, the offering is scattered in the beth hadeshen, and the scroll is not valid for the drinking of a different sotah. R. Achi b. R. Yoshiyah says: It is valid. "and he shall wave the offering": back and forth and up and down. Whence is this derived? From (Shemot 29:23) "which was waved and which was lifted": Lifting is hereby likened to waving. Just as waving is back and forth, so, lifting. And just as lifting is up and down, so, waving — whence they ruled: The mitzvah of waving — back and forth, up and down. "before the L-rd": in the east (i.e., at the eastern side of the altar [opposite the sanctuary]). Wherever "before the L-rd" is written, the east is intended unless specified otherwise. "and he shall wave the offering before the L-rd, and he shall present it at the (south-west corner of the) altar" — whereby we are taught that the offering of the sotah requires waving and presentation. (Ibid. 26) "And the Cohein shall take a fistful from the offering as its 'remembrance,' and he shall smoke it on the altar.": This refers to the smoking of the fistful, which is called "remembrance" (viz. Vayikra 2:2) "and then he shall make the woman drink the water": as mentioned above.