578 – Umm Salamah (may Allah be pleased with her) narrated: “The Messenger of Allah — peace and blessings be upon him — most often fasted on Saturday and Sunday, and he used to say, ‘They are festival days for the polytheists, and I wish to differ from them.’” Recorded by al-Nasa’i, and Ibn Khuzaymah declared it authentic; this is his wording. (1)

Degree of the ḥadith:
The ḥadith is weak.
It was also transmitted by Ibn Ḥibbān (3616) and al-Ḥākim (1593), who said, “Its chain is sound,” and al-Dhahabī concurred. ʿAbd al-Ḥaqq al-Ishbīlī, however, judged this chain weak because it contains a narrator whose status is unknown. Even if it were authentic, it cannot abrogate the report of al-Ṣammāʾ bint Busr, nor stand in opposition to it, for it may be interpreted as meaning that he fasted Friday along with Saturday, thereby not singling Saturday out. Hence Ibn ʿAbd al-Hādī said after citing the ḥadith of Ibn ʿAbbās: “This does not contradict the prohibition of fasting Saturday alone.” Our teacher—meaning Ibn Taymiyyah—said: “The ḥadith provides no evidence for singling out Saturday for fasting.” Allah knows best.

Points derived from the two ḥadiths:

1. Ḥadith 577 indicates the dislike of fasting Saturday by itself; this applies when it is not part of one’s regular routine of fasting, nor a fast made up for a missed day, nor a vowed or expiatory fast.
2. The wisdom behind this prohibition, and Allah knows best, is that Saturday is a day venerated by the Jews: they refrain from worldly tasks and devote themselves to worship. Fasting on it would resemble their veneration, and imitating the disbelievers is forbidden, for “whoever imitates a people is counted among them.”