Related 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.

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