Adam Clarke on Deuteronomy 31:16

Behold, thou shalt sleep with thy fathers - שכב shocheb, thou shalt lie down; it signifies to rest, take rest in sleep, and, metaphorically, to die. Much stress cannot be safely laid on this expression to prove the immortality of the soul, or that the people in the time of Moses had a distinct notion of its separate existence. It was, however, understood in this sense by Jonathan ben Uzziel, who in his Targum paraphrases the word thus: "Thou shalt lie down in the dust with thy fathers; and thy soul (נשמתך nishmethach) shall be laid up in the treasury of the life to come with thy fathers."