Examine carefully the speech of everyone you hear...

May Allaah have mercy upon you! Examine carefully the speech of everyone you hear from in your time particularly. So do not act in haste and do not enter into anything from it until you ask and see: Did any of the Companions of the Prophet, sallAllaahu 'alayhi wa sallam, speak about it or any of the scholars? So if you find a narration from them about it, cling to it, and do not go beyond it for anything and do not give precedence to anything over it and thus fall into the Fire.

Explanation of the Creed - Imaam al-Barbahaaree (rahimahUllaah)

Footnotes:

Imaam al-Awzaa'ee, rahimahUllaah, said, 

"Knowledge is what comes from the Companions of Muhammad (sallAllaahu 'alayhi wa sallam) and that which does not come from a single one of them is not knowledge." 

[Jami' Bayaanil-'Ilm of Ibn 'AbdulBarr 2/36]