בראשית פרק יט , לג וַתַשְ קֶּיןָ אֶּת -אֲבִיהֶּן יַיִן בַלַיְלָה הוּא וַתָבֹּא הַבְכִירָה
מסכת נזיר דף כג/ב א"ר חייא בר אבין א"ר יהושע בן קרחה לעולם יקדים אדם לדבר מצוה שבשכר לילה אחת שקדמתה בכירה לצעירה זכתה וקדמה ארבעה דורות בישראל למלכות :
This Gemara relies on the Medrjash’s understanding that Lot’s daughters’ intention was for good. They believed that the world’s population was destroyed and their obligation was to repopulate the world. Therefore, we can learn this principle of the reward to the first to do a mitzvah, since the eldest daughter (mother of Moav) was rewarded since she was first. The reward for this act of being first occurred after Rus (Moavis) converted, had a child and began a Moavi line of Jewish Kings with her son Oved; whereas the Amonis descendant began Amoni line of Jewish kings with Rehoboam, son of Shlomo and an Amonis, four generations later (after Oved, Yishai, Dovid, and Shlomo).
