The minimal level of completeness in glorifying Allah during bowing and prostration is to say the tasbih three times. If the congregants actually prefer a longer prayer, their number is limited, and they are unanimous in requesting the lengthening, then there is no harm in the imam prolonging it, for the element of harm—namely driving worshippers away—has been removed.
Imam Ibn Daqiq al-ʿId said: “The jurists’ statement, ‘The imam should not add more than three tasbihs in bowing and prostration,’ does not contradict the reports that the Prophet—peace and blessings be upon him—sometimes exceeded that, because the Companions’ eagerness for virtue meant that such lengthening was not burdensome.”
Shaykh al-Islam Ibn Taymiyyah remarked: “He is not permitted to go beyond what has been prescribed, and he should for the most part do what the Prophet usually did, increasing or decreasing whenever a genuine benefit requires it, just as the Prophet—peace and blessings be upon him—would occasionally lengthen or shorten for the sake of benefit.”
Al-Nawawi said: “The scholars have explained that the varying lengths of Qur’an recitation mentioned in the hadiths were dictated by circumstances. At times the Prophet—peace and blessings be upon him—knew that the believers preferred a longer prayer, so he lengthened it for them; at other times they did not prefer it because of an excuse, so he abbreviated it; and at yet other times he intended to lengthen but then heard a child crying, so he shortened it, as is authentically established.”
It is reprehensible for the imam to shorten the prayer to such an extent that the followers are unable