צַו אֶת אַהֲרֹן וְאֶת בָּנָיו לֵאמֹר זֹאת תּוֹרַת הָעֹלָה הִוא הָעֹלָה עַל מוֹקְדָה עַל הַמִּזְבֵּחַ כָּל הַלַּיְלָה עַד הַבֹּקֶר וְאֵשׁ הַמִּזְבֵּחַ תּוּקַד בּוֹ
Command Aaron and his sons saying, "This is the law of the burnt offering that goes up on its firewood on the altar all night until the morning; the fire of the altar shall be kept burning on it."
The preliminary morning service describes the burnt offering and the removal of the ashes. All over the Torah, Moshe is instructed to “tell” things to Aharon. Why here is he told to “command” Aharon? Rashi cites the Sifra that the kohanim might need a little extra encouragement to perform this mitzvah. They made their living from portions they received of the various sacrifices. Since the korban olah was burned in its entirety, it might feel “wasteful” to them and make them more reluctant to carry out this particular task.