{It is said} (eir(8874)ai). Perfect passive indicative, stands
said, a favourite way of quoting Scripture in the N.T. In  Mt
4:7  we have the usual "it is written" (gegraptai). Here Jesus
quotes  De 6:16 . Each time he uses Deuteronomy against the
devil. The LXX is quoted. It is the volitive future indicative
with ouk, a common prohibition. Jesus points out to the devil
that testing God is not trusting God (Plummer).