Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 12
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 12, with English translation by Maimonides (Rambam).
לִקְשֹׁר תְּפִלִּין בָּרֹאשׁ, שֶׁנֶּאֱמָר: "וְהָיוּ לְטֹטָפֹת בֵּין עֵינֶיךָ" (דברים ו, ח).
To tie tefillin upon our heads, as [Deuteronomy 6:8] states: "And they shall be an emblem between your eyes."