וְאִם־לֹ֨א תִשְׁמְע֜וּ אֵלַ֗י לְקַדֵּשׁ֙ אֶת־י֣וֹם הַשַּׁבָּ֔ת וּלְבִלְתִּ֣י ׀ שְׂאֵ֣ת מַשָּׂ֗א וּבֹ֛א בְּשַׁעֲרֵ֥י יְרוּשָׁלִַ֖ם בְּי֣וֹם הַשַּׁבָּ֑ת וְהִצַּ֧תִּי אֵ֣שׁ בִּשְׁעָרֶ֗יהָ וְאָֽכְלָ֛ה אַרְמְנ֥וֹת יְרוּשָׁלִַ֖ם וְלֹ֥א תִכְבֶּֽה׃ (פ)
你們若不聽從我，不以安息日為聖日，反而在安息日擔負重擔，進入耶路撒冷的各城門，我就必在城門中點起火來，這火必燒毀耶路撒冷的宮殿，總不熄滅。
Shenei Luchot HaBerit
When we read in 31,16: לעשות את השבת, the extra word את suggests that we should add a little from the weekday to the Sabbath, i.e. commence the Sabbath before sunset. The words: לא תבערו אש, "Do not make a fire" in 35,3, also hint at the statement in the Talmud that wherever we find that the Sabbath is also being desecrated, we are liable to find a conflagration (Midrash Hagadol based on Jeremiah 17,27). Observance of the Sabbath on the other hand, will lead to the redemption and the rebuilding of the Holy Temple. G–d will then be as a wall of fire for Zion, as we know from Zachariah 2,9.