Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) said: "The cupper and the cupped have broken their fast."
(Mahmud bin Ghaylan narrated from Qabisah that he narrated it from his book, from Fitr, from Ata', in mursal form, and that is the preserved (mahfuz) version; the mention of Ibn Abbas in it is an error.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8290