Reference for Shabbat 123:4
התם משום שי"ן דאמר אביי שי"ן של תפילין הלכה למשה מסיני ואמר אביי ד' של תפילין הלכה למשה מסיני ואמר אביי יו"ד של תפילין הלכה למשה מסיני:
Abaye also said: The daleth of tefillin is a <i>halachah</i> of Moses at Sinai. Abaye also said: The yod of tefillin is a <i>halachah</i> of Moses at Sinai.<span class="x" onmousemove="('comment',' The strap of the head-phylactery is knotted at the back of the head in the shape of a daleth ([H]); that of the hand-phylactery forms a noose and is knotted near the capsule in the shape of a yod ([H]). Cf. Heilprin. Seder ha-Doroth, I, p. 208 ed. Maskileison. Warsaw, 1897. Thus the three together make up the word [H] = Almighty. Tosaf., however, s.v. [H], deletes Abaye's last two statements on the daleth and yod. ');"><sup>4</sup></span>