Jeremiah 33:25

כֹּה אָמַר ה' אִם לֹא בְרִיתִי יוֹמָם וָלָיְלָה חֻקּוֹת שָׁמַיִם וָאָרֶץ לֹא שָׂמְתִּ

Thus says G-d: If not for My covenant day and night, I would not have set the laws of heaven and earth.

Rashi references the Sages of the Talmud, who say that the covenant of the day and night refer to the covenant of Torah and the covenant of circumcision. As the Talmud explains, Torah must be studied day and night. Also, once circumcised, a person carries the sign of that covenant with him both day and night. (See Pesachim 68b for the discussion of Torah; Mishna Nedarim 3:11 for that of circumcision.)