Midrash su Geremia 33:19

The great light alludes to the light created on the first day of creation, which the Holy One, blessed be He, reserved for those who devote themselves to the study of the Oral Law day and night. It is as a consequence of their merit that the world endures, as it is said: Thus saith the Lord: If My covenant be not with day and night, if I have not appointed the ordinances of heaven and earth (Jer. 33:25). Which covenant applies to day and night? It is the covenant of the Oral Law, as it is said: Thus saith the Lord: If you can break My covenant with the day and My covenant with the night … then may also My covenant be broken with David, My servant (ibid., vv. 20–21), and it says elsewhere: His delight is in the law of the Lord, and in His Law doth he meditate day and night (Ps. 1:2).