Sa'd bin Hudhayfah relates: On the day of the Battle of Siffin, Sayyiduna Ammar bin Yasir (may Allah be pleased with him) spoke; mentioning the affair of those people and the affair of the peace settlement, he said: By Allah, these people did not become Muslims; they displayed Islam and kept disbelief hidden, and when they found helpers for it, they brought it into the open.
Al-Tabarani recorded it in al-Mu'jam al-Kabir; I have not seen anyone give a biography of the narrator named Sa'd bin Hudhayfah.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الإيمان / 439