Musnad Ahmad 85
Asim bin Kulaib said:
My father said: I told Ibn 'Abbas and he said: What is so amazing about that? When ʼUmar called the prominent shaikhs among the Companions of Muhammad ﷺ, he would call me with them and say: Do not speak until they have spoken. He called us one day or one night and said: The Messenger of Allah ﷺ  said about Lailatal-Qadr what you know, so seek it in the odd numbered nights of the last ten nights; on which odd-numbered night do you think it is?
حَدَّثَنَا عَفَّانُ، حَدَّثَنَا عَبْدُ الْوَاحِدِ بْنُ زِيَادٍ، قَالَ حَدَّثَنَا عَاصِمُ بْنُ كُلَيْبٍ، قَالَ قَالَ أَبِي فَحَدَّثَنَا بِهِ ابْنُ عَبَّاسٍ، رَضِيَ اللَّهُ عَنْهُمَا قَالَ وَمَا أَعْجَبَكَ مِنْ ذَلِكَ كَانَ عُمَرُ رَضِيَ اللَّهُ عَنْهُ إِذَا دَعَا الْأَشْيَاخَ مِنْ أَصْحَابِ مُحَمَّدٍ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ دَعَانِي مَعَهُمْ فَقَالَ لَا تَتَكَلَّمْ حَتَّى يَتَكَلَّمُوا قَالَ فَدَعَانَا ذَاتَ يَوْمٍ أَوْ ذَاتَ لَيْلَةٍ فَقَالَ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ فِي لَيْلَةِ الْقَدْرِ مَا قَدْ عَلِمْتُمْ فَالْتَمِسُوهَا فِي الْعَشْرِ الْأَوَاخِرِ وِتْرًا فَفِي أَيِّ الْوِتْرِ تَرَوْنَهَا‏.‏