Where in the Bible does it say that wheat flour alone is valid for the Communion host?
The Bible does not explicitly mandate wheat flour for the Communion host. The Catholic Church, the Church Jesus established to teach in his name (cf. Matt. 16:18-19, 28:19-20; Luke 10:16), does mandate wheat flour for the Eucharist in its Code of Canon Law:
§1 The most holy sacrifice of the Eucharist must be celebrated in bread, and in wine to which a small quantity of water is to be added.
§2 The bread must be wheaten only, and recently made, so that there is no danger of corruption.
§3 The wine must be natural, made from grapes of the vine, and not corrupt
(canon 924; emphasis added).
Although the Bible does not explicitly state that only wheat flour may be used for the eucharistic hosts, it does support the Church’s mandate in many places. For more information, please see the article linked below.
I Would Feed You with the Finest of Wheat** by David P. Lang