Musnad Ahmad 2:58

Yunus ibn Muhammad narrated to us, Dawud—meaning Ibn Abi al-Furat—narrated to us, from Abdullah ibn Buraydah, from Abu al-Aswad, that he said: I came to Medina and arrived while a disease had broken out in it, and they were dying in great numbers. I sat with Umar ibn al-Khattab, may Allah be pleased with him, and a funeral passed by him, and its owner was praised with good. Umar said: "It has become incumbent." Then another passed by, and its owner was praised with good, and Umar said: "It has become incumbent." Then a third passed by, and it was praised with evil, and Umar said: "It has become incumbent." Abu al-Aswad said: "What has become incumbent, O Commander of the Faithful?" He said: "I said as the Messenger of Allah, peace and blessings of Allah be upon him, said: 'Any Muslim for whom four bear witness to goodness, Allah will cause him to enter Paradise.'" He said: "So we said: 'And three?'" He said: "He said: 'And three.'" He said: "We said: 'And two?'" He said: "And two." He said: "Then we did not ask him about one."