'It is Written' in the Bible

And Jehoiada appointed the offices of the house of Jehovah under the hand of the priests, the Levites, whom David had set by classes over the house of Jehovah to offer up Jehovah's burnt-offerings, as it is written in the law of Moses, with rejoicing and with singing according to the directions of David.

And he ordered that the king's portion should be taken from his substance for the burnt-offerings: for the morning and evening burnt-offerings, for the burnt-offerings of the sabbaths, and of the new moons, and of the set feasts, as it is written in the law of Jehovah.

And they set apart the burnt-offerings to give them to the classes of the fathers' houses of the children of the people, to present them to Jehovah, as it is written in the book of Moses. And so did they with the oxen.