לֹ֣א תֹאסִפ֞וּן לָתֵ֨ת תֶּ֧בֶן לָעָ֛ם לִלְבֹּ֥ן הַלְּבֵנִ֖ים כִּתְמ֣וֹל שִׁלְשֹׁ֑ם הֵ֚ם יֵֽלְכ֔וּ וְקֹשְׁשׁ֥וּ לָהֶ֖ם תֶּֽבֶן׃
Rashi on Exodus
תבן STRAW — In old French estouble; English stubble. They used to knead this into the clay.
Rashbam on Exodus
לתת תבן, the straw was mixed in with the mud to make the mud bricks firmer and less likely to crumble.
Rabbeinu Bahya
לא תאספון לתת תבן, “do not continue to supply straw, etc.” It is entirely possible that this decree was in force for only a single day and that the unusual letter א in the word תאספון is proof of this. This is the view of Rabbeinu Chananel.