Charles H. Spurgeon on Psalms 121:4

He... shall neither slumber nor sleep. This form of expression, he will not slumber nor sleep, would be improper in other languages, according to the idiom of which it should rather be, He will not sleep, yea, he will not slumber: but when the Hebrews invert this order, they argue from the greater to the less. The sense then is, that as God never slumbers even in the smallest degree, we need not be afraid of any harm befalling us while he is asleep. —John Calvin.