Musnad Ahmad 620:1

'Affan narrated to us, Aban narrated to us, Yahya ibn Abi Kathir narrated to us, from Zayd, from Abu Salam, from a freed slave of the Messenger of Allah (peace and blessings of Allah be upon him), that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Bakh, bakh (an expression of admiration) for five things! How heavy they are in the scale: 'La ilaha illa Allah' (There is no god but Allah), 'Allahu Akbar' (Allah is the Greatest), 'Subhan Allah' (Glory be to Allah), 'Al-hamdu lillah' (Praise be to Allah), and a righteous child who dies and his father seeks reward for him [from Allah]." And he said: "Bakh, bakh for five things; whoever meets Allah with certainty in them will enter Paradise: believing in Allah, the Last Day, Paradise and Hell, the Resurrection after death, and the Reckoning."