ולבלתי סור מן המצוה. אֲפִלּוּ מִצְוָה קַלָּה שֶׁל נָבִיא: 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. מִכְּלַל הֵן אַתָּה שׁוֹמֵעַ לָאו; וְכֵן מָצִינוּ בְשָׁאוּל שֶׁאָמַר לוֹ שְׁמוּאֵל (שמואל א י') "שִׁבְעַת יָמִים תּוֹחֵל עַד בּוֹאִי אֵלֶיךָ", לְהַעֲלוֹת עוֹלוֹת, וּכְתִיב (שם י"ג) "וַיּוֹחֶל שִׁבְעַת יָמִים", וְלֹא שָׁמַר הַבְטָחָתוֹ לִשְׁמֹר כָּל הַיּוֹם, וְלֹא הִסְפִּיק לְהַעֲלוֹת הָעוֹלָה עַד שֶׁבָּא שְׁמוּאֵל וְאָמַר לוֹ "נִסְכָּלְתָּ לֹא שָׁמַרְתָּ וְגוֹ' וְעַתָּה מַמְלַכְתְּךָ לֹא תָקוּם", הָא לָמַדְתָּ שֶׁבִּשְׁבִיל מִצְוָה קַלָּה שֶׁל נָבִיא נֶעֱנַשׁ: 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. מַגִּיד שֶׁאִם בְּנוֹ הָגוּן לְמַלְכוּת הוּא קוֹדֵם לְכָל אָדָם (הוריות י"א): 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).