“SAID Rabbi Chananel in the name of Rav, ‘the titura (bridge) of the tefilin (phylacteries) is halachah le-Moshe mi-Sinai’ [oral tradition going back to divine revelation to Moses at Mt. Sinai].
“Said Abbaye, ‘the ma’avarta (passage or channel) of the tefilin is halachah le-Moshe mi-Sinai.’
“And Abbaye said, ‘the shin (letter) of the tefilin is halachah le-Moshe mi-Sinai.’
“It was taught, square tefilin are halachah le-Moshe mi-Sinai.’
“Said Rabbi Yitzchak, black straps are halachah le-Moshe mi-Sinai.’”