Abu Dawud (may Allah have mercy on him) said: All the hadiths from the Prophet (peace and blessings be upon him) contain: "Let him give the expiation for his oath," except what is not worth taking into account.
Abu Dawud said: I said to Ahmad bin Hanbal: Yahya bin Sa'id narrated from Yahya bin Ubaydullah. He said: He abandoned him after that, and he deserved it. Ahmad bin Hanbal said: His hadiths are munkar (rejected), and his father is unknown.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19861