Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1889
Abu Wa'il reported: 'Ammar delivered to us the sermon. It was short and eloquent. When he (, Ammir) descended (from the pulpit) we said to him: 0 Abd al-Yaqzn, you have delivered a short and eloquent sermon. Would that you had lengthened (the sermon). He said: I have heard the Messenger of Allah (may peace be upon him) as saying: The lengthening of prayer by a man and the shortness of the sermon is the sign of his understanding (of faith). So lengthen the prayer and shorten the sermon, for there is charm (in precise) expression.