question #1 Sahih Muslim Introduction 62, Introduction, Narration 61 : What is the significance of trustworthiness in the transmission of hadiths?

‘I did not see Ayyūb speaking ill of anyone, ever, except for Abd al-Karīm- meaning Abū Umayyah’. So he mentioned him and said, may Allah have mercy on him ‘He is not trustworthy- he had asked me about a Ḥadīth of Ikrimah then said ‘I heard from Ikrimah’ [when relating the Ḥadīth].’