Sayyiduna Abdullah bin Zayd al-Ansari (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "When one of you is asked: 'Are you a believer?', he should not express doubt."
Al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains a narrator, Ahmad bin Budayl, whom al-Nasa'i (may Allah have mercy on him) and Abu Hatim declared thiqah (trustworthy), while others declared him da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الإيمان / 172