Musannaf Ibn Abi Shayba 23:24

Abu Mu’awiyah narrated to us, from al-Shaybani, from Tha’labah, from Abu Qilabah, who said: The messenger who [asked] Abdullah ibn Mas’ud narrated to me, saying: "I adjure you by Allah, do you know that the people in the era of the Messenger of Allah, may Allah bless him and grant him peace, were of three categories: a believer in secret and a believer in public, a disbeliever in secret and a disbeliever in public, and a believer in public and a disbeliever in secret?" He said: So Abdullah said, "O Allah, yes." He said: "Then I adjure you by Allah, which of them were you?" He said: "O Allah, a believer in secret and a believer in public; I am a believer." Abu Ishaq said: So I met Abdullah ibn Mughaffal and said, "Some people of righteousness criticize me for saying 'I am a believer'." Abdullah ibn Mughaffal said: "You have [failed] and lost if you are not a believer."