Sifrei Devarim 162

¹(Devarim 17:20) "So that his heart not rise above his brothers": and not (not rise) above idolators. ²"and so that he not turn aside from the mitzvah": even a "slight" mitzvah of a prophet. And thus is it written (I Samuel 13:13) "And Samuel said to Saul: You have acted foolishly. You have not kept the mitzvah, etc." "and so that he not turn aside from the mitzvah, right or left": even if he (seems to be) told that right is left and left is right. ³"so that he prolong his days in his kingdom": If he does as is written here, he will be worthy of prolonging days in his kingdom. ⁴"he and his sons": If he dies, his son succeeds him. This tells me only of a king. Whence do I derive (the same for) all of Israel's leaders? From "he and his sons in the midst of Israel" — All who are "in the midst of Israel" are succeeded by their sons. ⁵R. Chanina b. Gamliel says: It is written (I Kings 3:13) "And also what you (Solomon) did not ask I have granted you — even riches and honor": I have granted you even what is not stipulated in the Torah. Shall I not then grant you what is written in the Torah (i.e., length of days, etc.) Only on condition that you observe the Torah, viz. ( Ibid . 14) "If you (Solomon) walk in My ways to keep My statutes and My mitzvoth, as did your father David, then I shall prolong your days," and ( Ibid . 9:4) "And as for you, if you walk before Me as your father David did, with wholeheartedness and uprightness, to do according to all that I have commanded you, to keep My statutes and My judgments, etc."