Uqbah Bin Aamir (radiyallaahu-anhu) said: ”Learn before those who depend on guessing”; meaning: those who base their judgement on presumption. (1) Imaam Nawawi (rahimahullaah) stated in Al-Majmoo (1/42): The meaning of  it (i.e. this statement of Uqbah) is: ”Learn knowledge from its people before they depart -those competent god fearing people-and there appear a people (who) speak in (affairs) of knowledge by way of their own (ideas) and presumptions, which has no sharia basis.”  (2)

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[1] Saheeh Al-Bukhaari. Vol 8. Chapter: Learning about the laws of Inheritance

[2] Slightly paraphrased. For further details, see: Fadlul Ilm of Shaikh Raslaan. page:206