Albert Barnes Commentary Joshua 14:9

"And Moses sware on that day, saying, Surely the land whereon thy foot hath trodden shall be an inheritance to thee and to thy children for ever, because thou hast wholly followed Jehovah my God." — Joshua 14:9 (ASV)

Moses sware — that is, God swore; and His promise, confirmed by an oath, was communicated, of course, through Moses.