וַיִּפְקֹ֣ד מֹשֶׁ֔ה כַּאֲשֶׁ֛ר צִוָּ֥ה יְהוָ֖ה אֹת֑וֹ אֶֽת־כָּל־בְּכֹ֖ר בִּבְנֵ֥י יִשְׂרָאֵֽל׃
And Moses numbered, as the LORD commanded him, all the first-born among the children of Israel.
Read halakhic rulings (Jewish law) on Numbers 3:42: and classical halakhic sources.