ENOfficial
He reported God's messenger as saying, "People who arise from an assembly in which they did not remember God will be just as if they had got up from an ass's corpse, and it will be a cause of grief to them." Ahmad and Abu Dawud transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَا مِنْ قَوْمٍ يَقُومُونَ مِنْ مَجْلِسٍ لَا يَذْكُرُونَ اللَّهَ فِيهِ إِلَّا قَامُوا عَنْ مِثْلِ جِيفَةِ حِمَارٍ وَكَانَ عَلَيْهِمْ حَسرَةً» . رَوَاهُ أحمدُ وَأَبُو دَاوُد
sunnah.com