Musnad Ahmad 658:108

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Alqamah ibn Marthad, from Sa'd ibn 'Ubaydah, from al-Bara' ibn 'Azib, from the Prophet (peace and blessings of Allah be upon him), who mentioned the punishment of the grave. He said: "It is said to him: 'Who is your Lord?' He says: 'Allah is my Lord, and my Prophet is Muhammad.' That is the meaning of His saying: {Allah keeps firm those who believe, with the firm word, in the worldly life} [Ibrahim: 27]." He meant by that the Muslim.