(Bamidbar 28:16) "And in the first month, on the fourteenth day of the month, it is Pesach to the L-rd": Scripture (here) makes it mandatory (and not optional.) (Ibid. 28:17) "for seven days matzoth shall be eaten": I might think that any matzoth (may be eaten [e.g., even those made of rice]). It is, therefore, written (Devarim 16:3) "You shall not eat chametz with it. Seven days shall you eat matzoth with it." Only that which can be matzah (unleavened) or chametz (leavened) (is forbidden in the leavened state). These are the five varieties: wheat, barley, spelt, rye, or oats. This excludes rice, millet, sesame, paragim, which never become matzah or chametz, but only decay.
Sifrei Bamidbar 146
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