Musnad Ahmad 142:1

'Affan narrated to us, Aban narrated to us, Yahya ibn Abi Kathir narrated to us, from Zayd, from Abu Salam, from the 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 parents seek reward for him [from Allah]." And he said: "Bakh, bakh for five things; whoever meets Allah while being certain of them will enter Paradise: believing in Allah, the Last Day, Paradise, the Fire, and the Resurrection after death, and the Reckoning."