Talmud for Megillah 48:18
העושה תפלתו עגולה סכנה ואין בה מצוה: לימא תנינא להא דתנו רבנן תפלין מרובעות הלכה למשה מסיני ואמר רבא בתפרן ובאלכסונן
One who makes his tefillin [for the head] round, it is dangerous and has no religious value. Shall we say that that we have already taught this, as our Rabbis taught: [The obligation for] tefillin to be square is a law set down by Moses at Sinai. And Rava explained [this refers] to their stitching and their diagonal.
Jerusalem Talmud Megillah
HALAKHAH: Rebbi Yose ben Bevai stated: Square [black]153Corrector’s addition, of unknown source. The Babli only declares black strips as practice from Moses on Sinai. tefillin are practice from Moses on Sinai154Babli 24b, Šabbat 28b, Menaḥot 35a..
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