Amr bin Zurarah narrates that Abd al-Aziz narrated to us the same report as a brief mawquf hadith; and so Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) did not consider fasting obligatory upon the mu'takif.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8588