As it is written (hōs gegraptai). The regular formula for quotation, perfect passive indicative of graphō.
Isaiah the prophet (Esaiou tou prophētou). The same phrase in Mar 1:2 (correct text) and Mat 3:3. Mark, as we have seen, adds a quotation from Mal 3:1 and Luke gives Isa 40:4 and Isa 40:5 of Isa. 40 not in Matthew or Mark (Luk 3:5, Luk 3:6). See note on Mat 3:2; note on Mar 1:3 for discussion of Luk 3:4.