Imam Muslim said in the introduction to his 'Sahih': "To proceed, may Allaah have mercy upon you - if it were not for the evil practice that we have seen from many who take upon themselves the position of muhaddith in their leaving the obligation to discard the weak ahaadeeth and ‘munkar’ narrations and to suffice with only the authentic ahaadeeth – well known and transmitted by the reliable narrators, well known for their truthfulness and trustworthiness after knowing and admitting with their tongues that much of what they fling at the ignorant people is to be rejected, and is transmitted by unsatisfactory narrators whose narrations are censured by the scholars of hadeeth such as Maalik, Shu’bah, Yahyaa Ibn Sa’eed al Qattaan and others. So it has become easy for me to carry out what you ask regarding differentiating and gathering [ahaadeeth], because of what we have informed you of a people’s spreading ‘munkar’ narrations with weak and unknown isnaads ~ and flinging them to the common people who are unaware of their weakness - then due to this - what you have asked has become easy upon my heart.

And know! May Allaah ta’alaa grant you success – that what is obligatory upon everyone who is able to distinguish because authentic and weak narrations, and between the reliable and suspect narrators is that he should not narrate therefrom except that known to be authentic and have trustworthy narrators, and that he should avoid therefrom that narrated by narrators accused of lying, or wilful innovators..."