Musnad Ahmad 31:543

'Affan narrated to us, Wuhayb narrated to us, 'Amr ibn Yahya narrated to us, from his father, from Abu Sa'id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When the people of Paradise enter Paradise, and the people of the Fire enter the Fire, Allah, Blessed and Exalted is He, says: 'Whoever has in his heart the weight of a mustard seed of faith, then bring him out.' He said: So they are brought out, having been scorched and having become charcoal. Then they are cast into a river, which is called the River of Life, and they sprout in it just as a seed sprouts in the silt of a flood," or he said: "in the silt of the flood." Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do you not see that it sprouts yellow and twisted?"