What is the evidence that repeated oaths about the same thing, without expiation, require only one expiation?

Whoever repeats oaths for one thing and does not expiate for any of them, a single expiation is incumbent upon him for all those oaths. This is what the majority chose, and their evidence is the analogy of expiations to legal punishments (hudud) and purifications. Just as a single ablution suffices for various impurities, and a single punishment for repeated fornication, so too a single expiation suffices for repeated oaths, because expiation is a legal punishment and they overlap like legal punishments.