I know it was used in the 300 movie, and possibly in the 300 comic, but its earliest usage that I know of was:
Myth TFL was released in 1997, the 300 comic was published in 1998. Did Frank Miller take the line from Myth or is there an older usage?A berserk at the Stair of Grief, having been told that the hosts of the Soulless were so many that their spears would hide the sun, is said to have replied 'Then we shall fight them in the shade.'"
-Berserk Flavor Text, Myth TFL
Edit: Never-mind, found usages of the line attached to the Battle of Thermopylae that are much older.