"And when it is said to them: 'Believe as the people have believed'"—if the Prophet and the believers say to them: 'Believe as the believers have believed'—some of them say to others: "Shall we believe as the fools have believed?"—meaning: those who have believed; and they did not declare this statement of theirs. "Beware! It is they who are the fools, but they do not know" that they are fools; according to the tafsir (interpretation) of al-Hasan.
Muhammad said: The root of al-safah (foolishness) is the lightness of temperament; from it, it is said: A garment is safih (light/flimsy) when it is light. It is also said: The root of al-safah is ignorance.
﴿وَإِذَا قِيلَ لَهُمْ آمِنُوا كَمَا آمن النَّاس﴾ إِذا قَالَ لَهُم النَّبِي والمؤمنون: آمنُوا كَمَا آمن الْمُؤْمِنُونَ - قَالَ بَعضهم لبَعض: ﴿أَنُؤْمِنُ كَمَا آمن السُّفَهَاء﴾ يعنون: من آمن، وَلم يعلنوا قَوْلهم هَذَا ﴿أَلا إِنَّهُمْ هُمُ السُّفَهَاء وَلَكِن لَا يعلمُونَ﴾ أَنهم سفهاءُ؛ فِي تَفْسِير الحَسَن.
قَالَ مُحَمَّد: أصل السَّفه: خفَّة الْحلم؛ ومِنْهُ يُقَال: ثوب سَفِيه إِذا كَانَ خَفِيفا. وَقيل: أصل السَّفه: الْجَهْل.
