‘Alī b. Abī Khālid – Allāh have mercy on him – reports:
I once said to Aḥmad, “This shaykh – referring to an older man who was with us – is my neighbor. I told him not to keep the company of a certain person, and he would like to hear what you have to say about him: I am referring to Ḥārith Al-Qaṣīr (Al-Ḥārith Al-Muḥāsibī). Many years ago, you saw me with him and told me not to sit with him nor speak with him. I have not spoken to him since that time. This shaykh, however, does sit with him. So what do you say?”
I saw Aḥmad go red with anger, his eyes bulging; I had never before seen him like this. He started to say, “Him! May Allāh do such-and-such to him! Only those well-informed of him know what he really is, only those who really know him know what he is. Al-Mughāzilī, Ya’qūb and so-and-so sat with him, and he caused them to adopt the views of Jahm (Ibn Ṣafwān, leader of the Jahmites). They were destroyed because of him.”
The old man said, “But Abū ‘Abdillāh, he reports ḥadīth, and he is mild and humble; he has done such-and-such [good works].” Abū ‘Abdillāh (Imām Aḥmad) became angry and began repeating, “Let not his humility and softness deceive you”. He also said, “Do not be fooled by his bowed head, he is an evil man; only those well-informed of him through experience know him. Do not speak to him – with all disrespect to him. Are you going to sit with everyone who narrates from Allāh’s Messenger – may the praise and peace of Allāh be upon him – though he be a heretic (mubtadi’)? No, with all disrespect.”
Ṭabaqāt Al-Ḥanābilah, article 325.