As you do not specify book or movie, I will just do this the easy way and use Google…
According to IMDB, Subotai in the 1982 film uses: “How about two snakes coming together over a black sun? A magnificent standard!”
Wikiquotes has, also from the film:
Conan: A standard. A symbol. Perhaps on a shield. Two snakes coming together… Facing each other! But they’re one.
The Witch: With… With a sun and a moon below. Black Sun. Black Moon.
Relevant Google search was ‘conan the barbarian thulsa quote “two snakes”’