Musannaf Ibn Abi Shayba 7:314

Hafs narrated to us, from Layth, from Mujahid, from Ibn Abbas, who said: Everything in the Quran that contains "or" (aw), then one is given a choice in it. And everything in it that contains "then whoever does not find" (faman lam yajid), then it is the one that follows it; and if he does not find it, then the one that follows it.