Sahih Al Bukhari (565)

عن أبي مسعود قال {قال رجل يا رسول الله إني لأتأخر عن الصلاة في الفجر مما يطيل بنا فلان فيها فغضب رسول الله صلى الله عليه وسلم ما رأيته غضب في موضع كان أشد غضبا منه يومئذ ثم قال يا أيها الناس إن منكم منفرين فمن أم الناس فليتجوز فإن خلفه الضعيف والكبير وذا الحاجة}

Abu Mas`ud reported: A man came and said, "O Allah’s Messenger (peace be upon him)! I keep away from the morning prayer because so-and-so (Imam) prolongs it too much." Allah’s Messenger (peace be upon him) became furious and I had never seen him more furious than he was on that day. The Prophet (peace be upon him) said, "O people! Some of you make others dislike the prayer, so whoever becomes an Imam he should shorten the prayer, as behind him are the weak, the old and the needy.’’