Uthman bin Sa'id al-Darimi (may Allah have mercy on him) said: In my view, the hadith of "the cupper and the cupped" has been established as authentic on account of the hadith of Thawban and Shaddad (may Allah be pleased with them both), and I hold this view. And I heard Ahmad bin Hanbal hold this view and mention that the hadith of Thawban and Shaddad was authentic in his view.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8293