Rabbi Yudan said: The Torah has taught you proper conduct, that if a person has grown children, he should first marry them off, and only afterward take a wife for himself. From whom do you learn this? From Abraham. First it says, "And Isaac brought her into the tent of Sarah his mother" (Genesis 24:67), and only at the end, "And Abraham took another wife, and her name was Keturah" (Genesis 25:1).
Marry Off Your Children First as Abraham Did with Keturah
Yalkut Shimoni on Torah 109:14
אָמַר רַבִּי יוּדָן, לִמְדָתְךָ תּוֹרָה דֶּרֶךְ אֶרֶץ שֶׁאִם יִהְיוּ לְאָדָם בָּנִים גְּדוֹלִים יִהְיֶה מַשִּׂיאָן תְּחִלָּה וְאַחַר כָּךְ נוֹשֵׂא לוֹ אִשָּׁה. מִמִּי אַתָּה לָמֵד, מֵאַבְרָהָם, בִּתְחִלָּה וַיְבִיאֶהָ יִצְחָק הָאֹהֱלָה שָׂרָה אִמּוֹ, וּלְבַסּוֹף וַיֹּסֶף אַבְרָהָם וַיִּקַּח אִשָּׁה וּשְׁמָהּ קְטוּרָה.