Imam Shafi statement, “When you find a hadith that is Sahih then this is my madhab.”
Imam Nawawi (rahmatullah alayh) comments,
“This which Imam ash-Shaafi has said does not mean that everyone who sees a sahih Hadith should say “This is the mathhab of ash-Shaafi,” thus practicing on the zaahir (text/external or apparent meaning) of the Haditlh.
This most certainly applies to only such a person who has the rank of ijtihad in the madhhab. It is a condition that he overwhelmingly believes that Imam ash-Shaafi was unaware of this Hadith or he was unaware of its authenticity. And this is possible only after having made a research of all the books of ash-Shaafi and similar other books of the Ashaab of ash-Shaafi, those who take (knowledge) from him and others similar to these (books). This is indeed a difficult condition (to fulfill). Few are there who measure up to this (standard).
What we have explained has been made conditional because Imam ash-Shaafi had abandoned acting on the zaahir (text) of many Ahadith, which he say and knew. However, by him was established proof for criticism in the Hadith or its abrogation or it’s specific circumstance or its interpretation, etc. Hence, he was constrained to leave aside the hadith.”
(Ilaaus Sunan, Vol. 2, page 225)