In The Name of Allaah, The Most Merciful, The Bestowed of Mercy
Shaikh Uthaymeen [rahimahullaah] said:
Verification is one of the most important affairs, if not the most important. Verifying what is transmitted from others is an important affair, because sometimes the transmitters harbour evil intentions, so they transmit – intentionally and deliberately – that which will tarnish the reputation of the one they transmit from. And sometimes they do not harbour an evil intention, but they understand something in a manner that is opposite to what is intended by it.
Therefore, it is obligatory to verify. So, when the proof [sound source] of the transmission is established, then the one from whom the statement was transmitted is engaged in a discussion [or his statement is examined] before a judgement is passed to [establish] as to whether it is a mistake or not. That is because it may become clear to you – by way of discussion [or observation] – that the one from whom the statement was transmitted is correct.
And if not, then what is [well] known is that if a person were to immediately pass a judgement merely based on what he hears, he will transmit things [i.e. statements, views, opinions etc] from some of the scholars – those considered to be the beacons of [Sharee’ah] knowledge – that which the souls will dislike; but when one verifies, contemplates and attaches himself to the Shaikh [or has close contact with the Shaikh], he [i.e. the Shaikh] will make the affair clear for him.
Sharh Hilyati Taalibil Ilm’ pages 75-76.
abridged & slightly paraphrased