וּבִשְׁנַ֣ת עֶשְׂרִ֔ים לְיָרָבְעָ֖ם מֶ֣לֶךְ יִשְׂרָאֵ֑ל מָלַ֥ךְ אָסָ֖א מֶ֥לֶךְ יְהוּדָֽה׃
And in the twentieth year of Jeroboam king of Israel began Asa to reign over Judah.
Read halakhic rulings (Jewish law) on I Kings 15:9: and classical halakhic sources.