These scholars then differed over the underlying reason for the dislike.

Ibn ʿAqīl argued that Saturday is the day on which the Jews strictly observe their religion and set it aside for abstention—that is, they refrain from work on it. Because a person who is fasting is likewise inclined to cease working, fasting on that day would constitute imitating them. This rationale, he added, does not apply to Sunday.

Another group of Hanbalī jurists offered a different explanation: Saturday is a festival day for the People of the Book, which they venerate; therefore, singling it out for fasting would amount to venerating it as well. Hence it is disliked—just as singling out ʿĀshūrāʾ for special honor became disapproved when the People of the Book exalted it, and singling out Rajab was disapproved because the polytheists revered it. This reasoning, however, could be challenged with Sunday, for it is the Christians’ festival day, yet the Messenger of Allah (peace and blessings be upon him) said, “Today is ours; tomorrow is for the Jews; and the day after is for the Christians.”

One might therefore say: if the day in question is their festival, then opposing them should be achieved by fasting on it, not by refraining from fasting. Support for this is found in the report related by Kuraib, the freedman of Ibn ʿAbbās, who said:

“Ibn ʿAbbās, together with a number of the Prophet’s Companions, sent me to Umm Salamah to ask her, ‘Which days did the Messenger of Allah (peace and blessings be upon him) fast most frequently?’ She replied, ‘He used to fast on Saturday and Sunday more than on any other days, and he would say, “They are festival days for the polytheists, and I prefer to differ from them.”’”

This ḥadīth is reported by Aḥmad, al-Nasāʾī, and Ibn Abī ʿĀṣim, and some scholars declared it authentic.