מדרש על שמות 35:6

And his hands were steady until the going down of the sun (Exod. 17:12). Since time was calculated by means of the stars, what did Moses do? He stopped the revolution of the sun, the moon, and the coming of nightfall in their tracks, as it is said: The sun and the moon stand still in their habitation (Hab. 3:11). It is also written: The deep uttereth its voice, and lifteth up its hands on high (ibid., v. 10). And Joshua discomfited Amalek, etc. R. Joshua said: He went down and cut off the heads of their mighty men with a sword. Hence we learn that this battle was waged only at the command of the Mighty One. Others say that the following verse was fulfilled with regard to them: Therefore as I live, saith the Lord God, I will prepare thee unto blood, and blood shall pursue thee (Exod. 35:6).