Positive commandment · Mitzvat Aseh #13 of 613
To tie tefillin upon our arms
לִקְשֹׁר תְּפִלִּין בַּיָּד
The verse · Devarim 6:8
וּקְשַׁרְתָּם לְאוֹת עַל־יָדֶךָ וְהָיוּ לְטֹטָפֹת בֵּין עֵינֶיךָ
“And thou shalt bind them for a sign upon thy arm, and they shall be as frontlets between thy eyes.”
The Rambam’s words
To tie tefillin upon our arms, as [Deuteronomy 6:8] states: "And you shall tie them for a sign upon your arms."
לִקְשֹׁר תְּפִלִּין בַּיָּד, שֶׁנֶּאֱמָר: "וּקְשַׁרְתָּם לְאוֹת עַל יָדֶךָ" (דברים ו, ח).
Why · Sefer HaChinuch #421
Tefillin of the arm
“It is from the root of the commandment [that] since people are physical, they are necessarily drawn after their desires; for the nature of all physical things is to seek all which is comfortable for it and is pleasurable — “just as a horse or like a mule who understand nothing” — were it not that the soul with which G-d has graced us prevents us, to its ability, from sin.”
Who & when
“And it is practiced in every place and at all times by males, but not by females; because it is a positive commandment determined by time.”
Where in the Mishneh Torah
Hilchot Tefillin, Mezuzah and the Torah Scroll הלכות תפילין ומזוזה וספר תורה