Sa'id al-Maqburi said, "I passed by Ibn 'Umar who had a man with him with whom he was conversing. I went to them, and he struck me on the chest and said, 'When you find two men conversing, do not go up to them nor sit with them until they give you permission.' I said. 'May Allah make you prosper, Abu 'Abdu'r-Rahman. I hoped that I would hear something good from you.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدٌ، قَالَ: أَخْبَرَنَا عَبْدُ اللهِ، قَالَ: أَخْبَرَنَا دَاوُدُ بْنُ قَيْسٍ قَالَ: سَمِعْتُ سَعِيدًا الْمَقْبُرِيَّ يَقُولُ: مَرَرْتُ عَلَى ابْنِ عُمَرَ، وَمَعَهُ رَجُلٌ يَتَحَدَّثُ، فَقُمْتُ إِلَيْهِمَا، فَلَطَمَ فِي صَدْرِي فَقَالَ: إِذَا وَجَدْتَ اثْنَيْنِ يَتَحَدَّثَانِ فَلاَ تَقُمُّ مَعَهُمَا، وَلاَ تَجْلِسْ مَعَهُمَا، حَتَّى تَسْتَأْذِنَهُمَا، فَقُلْتُ: أَصْلَحَكَ اللَّهُ يَا أَبَا عَبْدِ الرَّحْمَنِ، إِنَّمَا رَجَوْتُ أَنْ أَسْمَعَ مِنْكُمَا خَيْرًا.
sunnah.com