Abū Wāʾil reports:
A man once came to ʿAbdullāh [ibn Masʿūd] – Allāh be pleased with him – and said, “We were once travelling when we met another group of travellers, so we asked them, ‘Who are you?’ They replied, ‘We are the believers!'” So [Ibn Masʿūd] said, “So why didn’t they just say, ‘We are from the people of Jannah?!'”
Abū ʿUbayd Al-Qāsim b. Sallām, Kitāb Al-Īmān, article 11. Shaykh Al-Albānī graded the chain of transmission of this and the following narration ṣaḥīḥ according to the standard of Bukhārī and Muslim.
A man once said to ʿAbdullāh [ibn Masʿūd], “I am a believer.” ʿAbdullāh said to him, “Why don’t you just say ‘I am in Jannah’! No, but [we say]: we believe in Allāh, his angels, his scriptures and his Messengers.”
Op. cit. article 12.