Musannaf Ibn Abi Shayba 23:25

Abu Mu'awiyah narrated to us, from Musa bin Muslim al-Shaybani, from Ibrahim al-Taymi, who said: "What is it upon any of you to say, 'I am a believer'? For by Allah, if he is truthful, Allah will not punish him for his truthfulness; and if he is a liar, that which has entered into him of disbelief is more severe upon him than the lie."