'Ali appointed Abu Mas'ud over the people, then went out on the day of 'Eid and said: 'O people, it is not part of the sunnah to pray before the imam.'" (Using translation from Nasāʾī 1561)  

أَنَّ عَلِيًّا اسْتَخْلَفَ أَبَا مَسْعُودٍ عَلَى النَّاسِ فَخَرَجَ يَوْمَ عِيدٍ فَقَالَ «يَا أَيُّهَا النَّاسُ إِنَّهُ لَيْسَ مِنَ السُّنَّةِ أَنْ يُصَلَّى قَبْلَ الْإِمَامِ»