It is said that as many days as there are in the whole journey, so many are the men and horses that stand along the road, each horse and man at the interval of a day’s journey; and these are stayed neither by snow nor rain nor heat nor darkness from accomplishing their appointed course with all speed.
Notes from Wikiquote
Paraphrase: "Neither snow, nor rain, nor heat, nor gloom of night stays these couriers from the swift completion of their appointed rounds" ” | Appears carved over entrance to Central Post Office building in New York City.
Variants
Not snow, no, nor rain, nor heat, nor night keeps them from accomplishing their appointed courses with all speed. (Book 8, Ch. 98)