Ibn Rajab said:
“As for the people of knowledge and understanding and those who adhere to the sunnah and the Jamaa’ah, then indeed they only mention the defects of the hadeeth out of sincerity to the religion and in order to preserve and safeguard the Prophet’s sunnah and to identify what befell the narrators of hadeeth from error, forgetfulness, and weakness.
This type of criticism was not required for other than weak and defective hadeeths. Rather, in their view, this strengthened the authentic hadeeths, since they were free from these defects and errors. So these individuals are the ones who are truly aware of the sunnah of the messenger of Allaah. And they are the great intellectual critics who criticeze the hadeeth after having skillfully and efficiently examined them in order to discredit the fake from the authentic…
[Sharh ‘Ilal At-Tirmidhee, 2/808]
Elsewhere he said:
“The righteous Imaams went to great lengths in forsaking the weak sayings (opinions) of some of the scholars. And they refuted them with the highest degree of refutation, as Imaam Ahmad used to censure Aboo Thawr and others in their opinions that they were alone in saying. And he went to great extremes in refuting them in these opinions…
[Al-Farq Bayna An-Naseeha Wat-Ta’yeer, p.30]