Musnad Ahmad 28:274

'Affan narrated to us, Hammad—meaning Ibn Salamah—narrated to us, from Thabit, from Abu Ayyub, that Nawf and 'Abdullah ibn 'Amr—meaning Ibn al-'As—met. Nawf said: "If the heavens and the earth and whatever is in them were placed in one scale of the balance, and 'La ilaha illa Allah' (There is no god but Allah) were placed in the other scale, it would outweigh them. And if the heavens and the earth and whatever is in them were a layer of iron, and a man said: 'La ilaha illa Allah,' it would pierce through them until it reached Allah, the Almighty and Majestic." 'Abdullah ibn 'Amr said: "We prayed the Maghrib prayer with the Messenger of Allah (ﷺ). Those who left departed, and those who returned returned. Then he (ﷺ) came, and he had almost pulled up his garments from his knees. He said: 'Rejoice, O assembly of Muslims! This is your Lord, who has opened a door from the doors of the heaven, boasting of you to the angels. He says: "These are My servants; they have fulfilled an obligatory duty, and they are awaiting another."'"