Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 7
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 7, with English translation by Maimonides (Rambam).
לְהִשָּׁבַע בִּשְׁמוֹ, שֶׁנֶּאֱמָר: "וּבִשְׁמוֹ, תִּשָּׁבֵעַ" (דברים ו יג, י כ).
To swear in His name, as [Deuteronomy 10:20] states: "And you shall swear in His name."