Bara' bin 'Azib (May Allah be pleased with them) reported:
The Prophet (ﷺ) said, "When a believer is questioned in the grave, he testifies that, 'there is no true god except Allah and Muhammad is the Messenger of Allah.' About him the Words of Allah, the Exalted, are: 'Allah will keep firm those who believe, with the firm statement (The Testimony of Faith) in this world and the Hereafter)". (14:27)[Al-Bukhari and Muslim].
Arabic/English book reference: Book 1, Hadith 427