הלכה על ברכות 41:2

However there is no set time for this commandment for us in the Torah. Hence our rabbis are in doubt about the matter: Rambam, may his memory be blessed, wrote in his great composition (Mishneh Torah, Laws of Prayer and the Priestly Blessing 1:2) that it is a commandment to pray each day. But Ramban, may his memory be blessed, (on Sefer HaMitzvot, Mitzvot Ase 5) wrangled with him and said that the Torah did not command us to pray every day, and also not (not even) every week, and it does not specify a time about the thing at all. And [that is why] they, may their memory be blessed always say that prayer is rabbinic (Berakhot 21a). And he says doubtfully that the commandment [from the Torah] is to pray and to cry out in front of God, blessed be He at a time of distress. Rambam, himself, may his memory be blessed, also wrote that the number of prayers and the format of the prayers is not from Torah writ and that the Torah does not have a set time for prayer. Nonetheless the obligation of the Torah is to supplicate to God every day and to thank Him, since all of the governance is His, [as] is the ability to fulfill every request. To here [are his words]. And it appears that in that the central commandment of the Torah is this and no more, they, may their memory be blessed, established for the one who is in a dangerous place and is not able to stand and concentrate in prayer to say, "The needs of Your people, Israel, are great, etc." - as it appears in Berakhot 29a - so as to fulfill his obligation from the Torah.