صحيح مسلم

Bishr bin ul-Hakam al-Abdī narrated to me, he said, I heard Sufyān bin Uyaynah saying, they informed me on authority of Abī Aqīl, companion of Buhayyah, that a descendent of Abd Allah bin Umar was asked about something that he did not have knowledge about, so Yahyā bin Sa’īd said to him: ‘By Allah, indeed it is a grave matter that the likes of you, a descendent of two Imāms of guidance – meaning Umar and Ibn Umar – is asked about a matter and you have no knowledge of it’. So [al-Qāsim] said: ‘By Allah, more grave than that according to Allah, and to whoever reflects about Allah, is to speak without knowledge or to report on authority of one who is not trustworthy’. [Ibn Uyaynah] said that Abū Aqīl Yahyā bin al-Mutawakkil witnessed them both when they said that.