Midrasch zu Divrej Hajamim I 2:15

“He will be for you a restorer of life, and nourisher of your old age, as your daughter-in-law, who loves you, who is better to you than seven sons, has borne him” (Ruth 4:15).
“He will be for you a restorer of life, and nourisher of your old age...[who is better to you than seven sons].” Rabbi Yehuda and Rabbi Neḥemya, Rabbi Yehuda says: Than the seven patrilineal heads mentioned later: “Otzem the sixth, David the seventh” (I Chronicles 2:15).²⁵⁷The reference is to the seven sons of Yishai. Rabbi Neḥemya said: Than the seven who are specified here: Peretz, Ḥetzron, Ram, Aminadav, Naḥshon, Salmon, Boaz.

11 All the sevenths are always beloved. Above the seventh is beloved: Skies, skies of the skies, firmament, heavens, celestial realm, abode, and clouds. And it is written (Psalms 68:5), "extol Him who rides the clouds; the Lord is His name." In the lands, the seventh is beloved: Earth, ground, globe, valley, wilderness, oblivion and world. And it is written (Psalms 9:9), "And He judges the world with righteousness, judges the peoples with equity." In the generations, the seventh is beloved: Adam, Shet, Enosh, Keinan, Mahalalel, Yered and Chanoch. And it is written (Genesis 5:25), "And Chanoch walked with God." In the forefathers, the seventh is beloved: Avraham, Yitschak, Yaakov, Levi, Kehat, Amram and Moshe. And it is written (Exodus 19:3), "And Moshe ascended to God." In sons, the seventh is beloved, as it is stated (I Chronicles 2:15), "David was the seventh." In kings, the seventh is beloved: Shaul, Ish-boshet, David, Shlomo, Rechavam, Aviyah and Asa. And it is written (II Chronicles 14:10), "And Asa called out to the Lord." In years, the seventh is beloved, as it is stated (Exodus 23.11), "But in the seventh you shall let it rest and lie fallow." In sabbatical years, the seventh is beloved, as it is stated (Leviticus 25:10), "And you shall sanctify the fiftieth year." In days, the seventh is beloved, as it is stated (Genesis 2:3), "And God blessed the seventh day." In months, the seventh is beloved, as it is stated (Leviticus 23:24), "In the seventh month on the first day."