Positive commandment · Mitzvat Aseh #80 of 613
To redeem firstborn sons
לפדות בכור אדם
The verse · Bamidbar 18:15
כׇּל־פֶּטֶר רֶחֶם לְכׇל־בָּשָׂר אֲשֶׁר־יַקְרִיבוּ לַה׳ בָּאָדָם וּבַבְּהֵמָה יִהְיֶה־לָּךְ אַךְ פָּדֹה תִפְדֶּה אֵת בְּכוֹר הָאָדָם וְאֵת בְּכוֹר־הַבְּהֵמָה הַטְּמֵאָה תִּפְדֶּה
“Everything that opens the womb in all flesh, which they bring to the Lord, whether it be of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.”
The Rambam’s words
To redeem firstborn sons, as [Numbers 18:15] states: "However, you must surely redeem first-born humans."
לפדות בכור אדם, שנאמר: "אך פדה תפדה, את בכור האדם" (במדבר יח,טו).
Why · Sefer HaChinuch #392
Redeeming the firstborn of a man
“It is from the roots of this commandment that G-d, may He be blessed, wanted to make us merit to do a commandment with the beginning of His fruit, in order that all should know that everything is His; and that man has nothing in the world, except that which G-d, may He be blessed, apportions to us, in His kindness.”
Who & when
“This commandment is practiced in every place and at all times by male Israelites, but not by females.”
Where in the Mishneh Torah
Hilchot First Fruits and other Gifts to Priests Outside the Sanctuary הלכות ביכורים ושאר מתנות כהונה שבגבולין