הלכה על מנחות 75:8

And [also] that which they said (Gittin 45b) that only an Israelite can make tefillin and their straps. And the length of the strap of the arm is enough to surround the forearm in the place that they are placed and he ties it from there with the famous knot that needs to be in the shape of a [letter,] yod, and he stretches [it] until the middle finger and he wraps three rings around it on his finger and he ties it. And if it is longer than this, is is [still] fit (Mishneh Torah, Laws of Tefillin, Mezuzah and the Torah Scroll 3:12). And upon which place in the upper arm do we tie it? Upon the biceps - and that is the protruding flesh in the [upper arm] between the shoulder joint and the elbow joint - such that it comes out that when he puts his [upper arm] next to his ribs, the tefillin will be laying across from his heart, and it comes out that he will fulfill, "And these words shall be [...] upon your heart" (Deuteronomy 6:6). And [also] that which they said (Menachot 38a) that the tefillin of the arm does not impinge upon the tefillin of the head, and [that] of the head does not impinge upon that of the arm, because they are two [separate] commandments. And he recites the blessing on that of the head, 'about the commandment of tefillin'; and on that of the arm, 'to place the tefillin.' And about what are these words speaking? When he puts on one of them. But if he put both of them on together, he only recites one blessing; and that is 'to place the tefillin.'