Ali bin Sa'id al-Nasawi (may Allah have mercy on him) narrates: I heard Imam Ahmad bin Hanbal (may Allah have mercy on him) being asked: Which hadith is most authentic in your view concerning the breaking of the fast of "the cupper and the cupped"? He said: The hadith of Thawban (may Allah be pleased with him), through the narration of Yahya bin Abi Kathir (may Allah have mercy on him), from Abu Qilabah, from Abu Asma', from Thawban. It was said to Imam Ahmad bin Hanbal (may Allah have mercy on him): Then what about the hadith of Rafi' bin Khadij (may Allah be pleased with him)? He said: Ma'mar (may Allah have mercy on him) is alone in narrating it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8291