Abu Wa'il (may Allah have mercy on him) says that Sayyiduna Ammar (may Allah be pleased with him) delivered to us an eloquent and brief sermon. When he came down from the pulpit, we said: O Abu al-Yaqzan! You gave an eloquent and brief sermon; it would have been better had you lengthened your speech. He said: I heard the Messenger of Allah (peace and blessings be upon him) say: "A man's lengthy prayer and short sermon are a sign of his understanding. So lengthen the prayer and keep the sermon short; and some speech indeed has the effect of magic."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5763