"and warred with Israel in Refidim": R. Chaninah said: I asked this of R. Elazar in the Great College. How is "Refidim" to be understood? He answered: as is (i.e., as a place name). R. Chanina said further: I asked R. Elazar: Why are the first-born of asses (especially) to be redeemed, and not those of horses or of camels? It was an exigency of the time, for at that time they had only asses, there being no Israelite who did not bring up with him ninety asses laden with silver and gold.