Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.051B.

Abu Hurairah reports that the Prophet said: “If one of you leads the people in prayer, he should be “easy,” on them for among the people are the weak, sick, and aged. If one prays by himself, one may make it as long as one wishes.” This is related by the group. It is narrated from Anas that the Prophet sallallahu alehi wassalam said: “Sometimes I enter prayer and I intend to prolong it, but then I hear a child crying, and I shorten my prayer thinking of the distress of the child’s mother.”

Al-Bukhari and Muslim record that Anas said: “I have not prayed behind anyone who prayed a lighter salah or a more complete prayer than that of the Prophet sallallahu alehi wasallam.” Abu ‘Umar ibn Abdul Barr said: The scholars agree that it is preferable for an imam to make the prayer light while preserving the minimum without which salah is incomplete and without leaving off any part of the salah or shortening part of it [not performing it properly]. The Prophet sallallahu alehi wasallam prohibited the pecking like a crow. Once he saw a man who did not complete his ruku’ and he told him: ‘Go back and pray for you have not prayed.’ And he said: ‘Allah does not look to one who does not straighten his back during ruku’ and sujjud.’ I do not know of any difference of opinion among the scholars concerning the fact that it is preferred for an imam to be ‘easy’ on his followers while making the prayer properly. It is related that ‘Umar said: ‘Do not make people dislike Allah, by making the salah so long that it should become hard on those praying behind you.”‘