Jamieson, Fausset & Brown

Again--Anew the promise recurs. Translate as the Greek order is, "He limited a certain day, 'To-day.'" Here Paul interrupts the quotation by, "In (the Psalm of) David saying after so long a time (after five hundred years' possession of Canaan)," and resumes it by, "as it has been said before (so the Greek oldest manuscript, before, namely, Heb 3:7, Heb 3:15), To-day if ye hear His voice," &c. [ALFORD].