question #3 Sahih Muslim Introduction 35, Introduction, Narration 34 : What is the significance of verifying the reliability of narrators in the science of hadith?

‘I asked Sufyān ath-Thawrī , Shu’bah, Mālik, and Ibn Uyaynah about a man who is not reliable (Thabt) in Ḥadīth, and someone comes and asks me about him, and they said: ‘Inform [others] against him that he is not reliable’.’