Hadithcore
Musnad Ahmad, 259
sahih· Darussalam

It was narrated that Qais said:

I saw `Umar with a leafless palm branch in his hand, and he was telling the people to sit down, saying: Listen to the words of the successor of the Messenger of Allah (ﷺ). A freed slave of Abu Bakr, whose name was Shadeed, brought the document and read it to the people, He said: Abu Bakr says: Listen to and obey what is in this document, for by Allah I am very sincere towards you, Qais said: And I saw `Umar after that on the minbar.

حَدَّثَنَا وَكِيعٌ، عَنِ ابْنِ أَبِي خَالِدٍ، عَنْ قَيْسٍ، قَالَ رَأَيْتُ عُمَرَ رَضِيَ اللَّهُ عَنْهُ وَبِيَدِهِ عَسِيبُ نَخْلٍ وَهُوَ يُجْلِسُ النَّاسَ يَقُولُ اسْمَعُوا لِقَوْلِ خَلِيفَةِ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَجَاءَ مَوْلًى لِأَبِي بَكْرٍ رَضِيَ اللَّهُ عَنْهُ يُقَالُ لَهُ شَدِيدٌ بِصَحِيفَةٍ فَقَرَأَهَا عَلَى النَّاسِ فَقَالَ يَقُولُ أَبُو بَكْرٍ رَضِيَ اللَّهُ عَنْهُ اسْمَعُوا وَأَطِيعُوا لِمَا فِي هَذِهِ الصَّحِيفَةِ فَوَاللَّهِ مَا أَلَوْتُكُمْ قَالَ قَيْسٌ فَرَأَيْتُ عُمَرَ رَضِيَ اللَّهُ عَنْهُ بَعْدَ ذَلِكَ عَلَى الْمِنْبَرِ‏.‏

References1 variant
In-Book Reference
Book 2, Hadith 173