It is written = It standeth written. This is the Lord's first ministerial utterance; three times. Compare the last three (Joh_17:8, Joh_17:14, Joh_17:17). The appeal is not to the spoken voice (Mat_3:17) but to the written Word. Quoted from Deu_8:3. See App-107 and App-117.
Man. Greek. anthropos.App-123.
by = upon. Greek. epi.App-104.
alone = only.
word = utterance. out of = by means of, or through. Greek. dia.App-104. Mat_4:1. Note the connection of the "hunger" and the "forty" days here, and the same in Deu_8:3.