Leviticus 6:8 And יהוה spoke to Moses saying, 9 Command אֶת־ Aaron וְאֶת־ and his sons saying, זֹאת This is the Torah (law) of the burnt offering: It is the burnt offering, because it will be burning upon the altar all night until morning, in this way the fire of the altar will be kept burning. C-MATS
Question: How was the burnt-offering a sacrifice for the Kohen? In order to perform the sacrificial service, Kohanim must give up their regular means of earning a livelihood. This financial sacrifice is particularly acute in the case of a burnt-offering, from which the Kohanim receive nothing, since all of its meat is burned on the Altar. Even though its hide goes to the Kohanim, the price of the hide is hardly sufficient to make up for their loss of income. Chumash
Question: What commandment was given to the Kohen? There was a general commandment to keep the Altar fires burning at all times and at least one negative commandment not to extinguish the fire or to allow it to go out. Chumash