Sahih Muslim Book - 0, Hadith - 34

و حَدَّثَنِي بِشْرُ بْنُ الْحَكَمِ الْعَبْدِيُّ قَالَ سَمِعْتُ سُفْيَانَ بْنَ عُيَيْنَةَ يَقُولُ أَخْبَرُونِي عَنْ أَبِي عَقِيلٍ صَاحِبِ بُهَيَّةَ أَنَّ أَبْنَاءً لِعَبْدِ اللَّهِ بْنِ عُمَرَ سَأَلُوهُ عَنْ شَيْءٍ لَمْ يَكُنْ عِنْدَهُ فِيهِ عِلْمٌ فَقَالَ لَهُ يَحْيَى بْنُ سَعِيدٍ وَاللَّهِ إِنِّي لَأُعْظِمُ أَنْ يَكُونَ مِثْلُكَ وَأَنْتَ ابْنُ إِمَامَيْ الْهُدَى يَعْنِي عُمَرَ وَابْنَ عُمَرَ تُسْأَلُ عَنْ أَمْرٍ لَيْسَ عِنْدَكَ فِيهِ عِلْمٌ فَقَالَ أَعْظَمُ مِنْ ذَلِكَ وَاللَّهِ عِنْدَ اللَّهِ وَعِنْدَ مَنْ عَقَلَ عَنْ اللَّهِ أَنْ أَقُولَ بِغَيْرِ عِلْمٍ أَوْ أُخْبِرَ عَنْ غَيْرِ ثِقَةٍ قَالَ وَشَهِدَهُمَا أَبُو عَقِيلٍ يَحْيَى بْنُ الْمُتَوَكِّلِ حِينَ قَالَا ذَلِكَ.

It was narrated by Sufyan [bin 'Uyaynah] who said: "They informed me about Abu 'Aqil, the companion of Buhayyah; that a son of 'Abdullah bin 'Umar was asked about something of which he did not have any knowledge. Yahya bin Sa'eed said to him: 'I feel it is very grave that a man like you, who is the son of two Imam of guidance' - meaning 'Umar and Ibn 'Umar - 'can be asked about something of which he has no knowledge.' He said: 'By Allah, it is more serious than that before Allah and before anyone who has any knowledge of Allah, to speak without knowledge, or to narrate from someone who is not trustworthy.' Abu 'Aqil Yahya bin Al-Mutawakkil was present while the two of them said that."