Sunan an-Nasa'i 1561
سنن النسائي#1561Sahihأَخْبَرَنَا إِسْحَاقُ بْنُ مَنْصُورٍ، قَالَ أَنْبَأَنَا عَبْدُ الرَّحْمَنِ، عَنْ سُفْيَانَ، عَنِ الأَشْعَثِ، عَنِ الأَسْوَدِ بْنِ هِلاَلٍ، عَنْ ثَعْلَبَةَ بْنِ زَهْدَمٍ، أَنَّ عَلِيًّا، اسْتَخْلَفَ أَبَا مَسْعُودٍ عَلَى النَّاسِ فَخَرَجَ يَوْمَ عِيدٍ فَقَالَ يَا أَيُّهَا النَّاسُ إِنَّهُ لَيْسَ مِنَ السُّنَّةِ أَنْ يُصَلَّى قَبْلَ الإِمَامِ .
It was narrated from Tha'labah bin Zahdam that:'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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
In-book reference: 19:6