It is reported from Al-‘Abbās b. Al-Walīd that ‘Uqbah said:

I was once with Arṭa’ah b. Al-Mundhir when one of the people in the gathering said, “What do you say about a man who sits with the followers of the Sunnah and mixes with them, but when the followers of Bid’ah are mentioned he says, ‘Spare us from mentioning them, do not talk about them?’” Arṭa’ah said, “He is one of them, do not let him confuse you about his condition.” I felt this was strange, so I went to Al-Awzā’ī – and he used to clarify these matters when they came to him. He said, “Arṭa’ah is right, the matter is as he said; this person forbids talking about [Ahl Al-Bid’ah], so how can [people] be on guard against them if they are not exposed?”

Ibn ‘Asākir, Tārīkh Dimishq 8:15.