Al-Imām Abu Muhammad Ibn Tamīm Al-Hanbalī (رحمه الله ) said describing Imām Ahmad رحمه الله :
“He was harsh against ahlul-bid’ah and against the one who drew close to them if he did not abandon them, even if his ‘aqīdah was correct. Imām Ahmad boycotted Ali Al-Madīnī, Yahyā Ibn Ma’īn, Al-Husayn Al-Karabīsī – and this remained the case right up until Yahyā Ibn Ma’īn repented in front of him.”
(Tabaqāt al-Hanābilah of Abu Ya’lā 2/289, Tahrīm an-Nadhr fī Kutubil-Kalām p.60)