Leviticus 6:12

It shall not be put out — The fire coming down from heaven, was to be perpetually preserved, and not suffered to go out, partly that there might be no occasion or temptation to offer strange fire; and partly to teach them whence they were to expect the acceptance of all their sacrifices, even from the divine mercy, signified by the fire that came down from heaven which was an usual token of God's favourable acceptance. Every morning — Though the evening also be doubtless intended, yet the morning is only mentioned, because then the altar was cleansed, and the ashes taken away, and a new fire made. Thereon — Upon the burnt — offering, which thereby would be sooner consumed, that the way might be made for other sacrifices.