Deuteronomy 17:20

ולבלתי סור מן המצוה AND THAT HE DEPART NOT FROM THE COMMANDMENT — not even from a less important command given to him by means of a prophet. למען יאריך ימים TO THE END THAT HE MAY PROLONG HIS DAYS [IN HIS KINGDOM] — From the positive statement you may derive the negative (that if he does not fulfil the commandments his kingdom will not endure). And so, indeed, do we find in the case of Saul that Samuel said to him, (I Samuel 10:8) “Seven days shalt thou tarry, till I come unto thee” to offer burnt sacrifices, and it is written, (I Samuel 13:8) “and he tarried seven days”, but he did not keep his promise to wait the whole of the seventh day and had scarcely finished offering the burnt offering when Samuel came (I Samuel 10:10) and said to him (I Samuel 10:13-14) “Thou hast done foolishly; thou hast not kept [the commandment of the Lord thy God, which He commanded thee; for now would the Lord have established thy kingdom upon Israel forever;] but now thy kingdom shall not continue”. Thus you learn that for the neglect of an unimportant command given by means of a prophet he was severely punished. הוא ובניו [THAT HE MAY PROLONG HIS DAYS IN HIS KINGDOM] HE, AND HIS CHILDREN — This tells you that if his son is worthy of becoming king he has to be given preference to any other person (Horayot 11b).

לְבִלְתִּ֤י רוּם־לְבָבוֹ֙ מֵֽאֶחָ֔יו וּלְבִלְתִּ֛י ס֥וּר מִן־הַמִּצְוָ֖ה יָמִ֣ין וּשְׂמֹ֑אול לְמַ֩עַן֩ יַאֲרִ֨יךְ יָמִ֧ים עַל־מַמְלַכְתּ֛וֹ ה֥וּא וּבָנָ֖יו בְּקֶ֥רֶב יִשְׂרָאֵֽל׃ {ס}        

that his heart be not lifted up above his brethren, and that he turn not aside from the commandment, to the right hand, or to the left: to the end that he may prolong his days in his kingdom, he, and his children, in the midst of Yisra᾽el.