Musnad Ahmad 1060:8

Hassan ibn Musa narrated to us, Abdullah ibn Lahi'ah narrated to us, Abu Qabil narrated to us, from Abdullah ibn Nashir, from the tribe of Sari', who said: I heard Abu Rahm, the storyteller of the people of Ash-Sham, saying: I heard Abu Ayyub al-Ansari saying: The Messenger of Allah (peace and blessings of Allah be upon him) went out to them one day and said to them: "Indeed, your Lord has given me the choice between seventy thousand who will enter Paradise without reckoning, and the hidden treasure (al-khabi'ah) He has with Him for my nation." Some of his companions said to him: "O Messenger of Allah, what is that which your Lord, the Mighty and Majestic, has hidden?" The Messenger of Allah (peace and blessings of Allah be upon him) entered [his house], then came out while saying "Allahu Akbar" (God is the Greatest), and said: "Indeed, my Lord has added for me, with every thousand, seventy thousand, and the hidden treasure is with Him." Abu Rahm said: "O Abu Ayyub, what do you think the hidden treasure of the Messenger of Allah (peace and blessings of Allah be upon him) is?" The people rebuked him, saying: "What do you have to do with the hidden treasure of the Messenger of Allah (peace and blessings of Allah be upon him)?" Abu Ayyub said: "Leave the man alone. I will inform you about the hidden treasure of the Messenger of Allah (peace and blessings of Allah be upon him) as I think, or rather, as one who is certain: The hidden treasure of the Messenger of Allah (peace and blessings of Allah be upon him) is that he says: 'O Lord, whoever testifies that there is no god but Allah alone, with no partner, and that Muhammad is His servant and His Messenger, with his heart confirming his tongue, admit him into Paradise.'"