Daghfal bin Hanzalah relates: Fasting the month of Ramadan was obligatory upon the Christians. They had a king who fell ill, and he said: "If Allah grants me recovery, I will certainly add ten days (to it)." Then after him they had another king; he ate meat and fell ill, and he said: "If Allah grants me recovery, I will certainly add eight days (to it)." Then after him they had another king, and he said: "We will not leave out any of these days, but rather complete them, and we will keep our fast in the season of spring." So he did that, and it became fifty days.
Hadith Referenceالمعجم الكبير للطبراني / باب الدال / 4188