ʿĀʾishah related that when the Prophet (peace and blessings be upon him) felt the effects of illness he prayed sitting down, while the Companions, assuming that their ability to stand required them to do so, prayed behind him standing. He therefore signalled to them to sit.
When he finished the prayer he explained that the imam is not to be opposed but matched, so that full following and perfect imitation are achieved; hence the congregant is to pray sitting, even though he is able to stand, whenever his imam is unable to stand and therefore prays sitting.

Scholarly disagreement
The jurists have differed over the validity of an obligatory worshipper following an imam who is offering a voluntary prayer.
The Malikis, Hanafis, and the predominant view within the Hanbali school judge the prayer invalid, citing the present hadith: “The imam has only been appointed to be followed, so do not differ with him.” They argue that the worshipper’s prayer is obligatory whereas the imam’s is voluntary, a divergence of intention that constitutes one of the gravest forms of discrepancy, for all actions turn on intention.
Shafiʿi, al-Awzaʿi, and al-Tabari, however, hold such following to be valid, as does the second report from Imam Ahmad preferred by Ibn Qudamah, Shaykh al-Islam Ibn Taymiyyah, and Ibn al-Qayyim. Their evidence is the agreed-upon hadith of Muʿadh: “He would pray with the Prophet (peace and blessings be upon him), then go back and lead his people in that very prayer.” They also cite the report: “The Prophet (peace and blessings be upon him) led one group of his Companions in the prayer of fear for two rakʿahs, gave the salām, then led the other group for two rakʿahs and gave the salām,” recorded by Abu Dawud; in this second performance the Prophet was praying voluntarily.
Here, “do not differ with him” means: do not differ from him in the actions of the prayer.
Those who deem the prayer valid challenge the opposing camp, saying: you yourselves recognise the validity of an obligatory worshipper following someone praying voluntarily in certain cases despite the divergence of intention, which is precisely what you object to here—so your reasoning is internally inconsistent.
The scholars also disagreed about whether worshippers who are able to stand should nevertheless pray sitting behind an imam unable to stand. The Ẓāhiris, al-Awzaʿi, and Isḥāq ruled that the congregants should sit behind such an imam even though they are able to stand, basing their view on these two hadiths and on other reports that have come