Dughfal narrated from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "Fasting the month of Ramadan was obligatory upon the Christians. They had a king over them who fell ill, so they said: 'If Allah cures him, we will add eight days (to the fast).' Then after him another king came; he ate meat and fell ill, so they said: 'If Allah cures him, we will add eight more days.' Then after him another king came, and he said: 'We will not leave out any of these days, but will complete them, and we will keep our fast in the spring.' So he did that, and thus the fast became fifty days."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from Qatadah except Hisham, and Mu'adh is alone in narrating it."
Hadith Referenceالمعجم الأوسط / باب الميم / 8193
Hadith Gradingمحدثین:رجاله رجال الصحيح
Hadith Takhrij«قال الهيثمي: رجال إسنادهما رجال الصحيح «مجمع الزوائد ومنبع الفوائد، ج 3، ص 139»» «أخرجه الطبراني فى «الكبير» برقم: 4203»