Rashi on Deuteronomy 34:4/רש"י על דברים ל״ד:ד׳

1לאמר לזרעך אתננה הראיתיך TO SAY, TO THY SEED WILL I GIVE IT, I HAVE LET THEE SEE IT, i.e. in order that you may go and say to Abraham, to Isaac and to Jacob: The oath which the Holy One, blessed be He, swore to you — that oath He has fulfilled. This is the force of לאמר (Berakhot 18b), “to say it”, viz., that you may say it — for that reason have I let you see it. But it is a decree from Me that שמה לא תעבר THERE YOU SHALL NOT CROSS OVER; for were this not so, I would keep you alive even until you saw them planted and settled in it, and you would then go and tell them (the patriarchs).