Musnad Abu Ya'la al-Mawsili 140:52

Muhammad ibn Ghalib wrote to me in his own handwriting, saying: Al-Muthanna narrated to me—Abu Ya’la said: meaning my grandfather—narrated to us, from Abu Shihab, from Yazid al-Raqashi, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I will knock on the door of Paradise, and a door of gold with a ring of silver will be opened for me. The greatest light will meet me, so I will fall down prostrate, and I will be inspired with praises for Allah that no one before me has been inspired with. It will be said to me: 'Raise your head, ask and you shall be given, intercede and your intercession will be accepted, speak and you will be heard.' I will say: 'My Ummah.' It will be said to me: 'For whoever has in his heart the weight of a barleycorn of faith.' He said: Then I will prostrate a second time, and I will be inspired with the like of that. I will say: 'My Ummah.' It will be said to me: 'For you is whoever has in his heart the weight of a mustard seed of faith.' Then I will prostrate a third time, and I will be inspired with the like of that, and it will be said to me the like of that. Then I will raise my head and say: 'My Ummah.' It will be said to me: 'For whoever said "There is no god but Allah" sincerely.'"