Sahih Bukhari Volume 1, Book 11, Hadith Number 672

Narated By Abu Mas’ud : A man came and said, “O Allah’s Apostle! I keep away from the morning prayer because so-and-so (Imam) prolongs it too much.” Allah’s Apostle became furious and I had never seen him more furious than he was on that day. The Prophet 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.”