Tafsir of Al-Qamar 54:50

(And Our command is but one), meaning: Our affair is but a single act according to a method that does not differ, and a pace that does not multiply; it is bringing into existence without manipulation or secondary cause. Alternatively, it means: Our command is but a single word, which is His saying, the Almighty: "Be." Thus, the command is the opposite of the prohibition, and the "one" refers to the affairs. When He, the Almighty and Majestic, wills a thing, He says to it: "Be," and it is (as a blink of an eye), meaning in motion and speed. It is also said: This refers to the establishment of the Hour, and it is like His saying, the Almighty: "And the command of the Hour is not but as a blink of an eye."