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 (ﷺ) 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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنِي سُرَيْجُ بْنُ يُونُسَ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ عَبْدِ الْمَلِكِ بْنِ أَبْجَرَ، عَنْ أَبِيهِ، عَنْ وَاصِلِ بْنِ حَيَّانَ، قَالَ قَالَ أَبُو وَائِلٍ خَطَبَنَا عَمَّارٌ فَأَوْجَزَ وَأَبْلَغَ فَلَمَّا نَزَلَ قُلْنَا يَا أَبَا الْيَقْظَانِ لَقَدْ أَبْلَغْتَ وَأَوْجَزْتَ فَلَوْ كُنْتَ تَنَفَّسْتَ . فَقَالَ إِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " إِنَّ طُولَ صَلاَةِ الرَّجُلِ وَقِصَرَ خُطْبَتِهِ مَئِنَّةٌ مِنْ فِقْهِهِ فَأَطِيلُوا الصَّلاَةَ وَاقْصُرُوا الْخُطْبَةَ وَإِنَّ مِنَ الْبَيَانِ سِحْرًا " .
sunnah.com