וַיֹּ֧אמֶר שְׁמוּאֵ֛ל אֶל־שָׁא֖וּל נִסְכָּ֑לְתָּ לֹ֣א שָׁמַ֗רְתָּ אֶת־מִצְוַ֞ת יְהוָ֤ה אֱלֹהֶ֙יךָ֙ אֲשֶׁ֣ר צִוָּ֔ךְ כִּ֣י עַתָּ֗ה הֵכִ֨ין יְהוָ֧ה אֶת־מַֽמְלַכְתְּךָ֛ אֶל־יִשְׂרָאֵ֖ל עַד־עוֹלָֽם׃
Rashi on I Samuel
Adonoy would have established your kingship … forever. Because when a high position is assigned to a person, it is assigned to him and to his descendants.9 See Maseches Megillah 13b.