Sahih Muslim 184a

Harun ibn Sa‘id al-Ayli narrated to me: Ibn Wahb narrated to us: Malik ibn Anas informed me, from ‘Amr ibn Yahya ibn ‘Umara, who said: my father narrated to me, from Abu Sa‘id al-Khudri, that the Messenger of Allah (peace and blessings be upon him) said: “Allah will admit the people of Paradise into Paradise, admitting whomever He wills by His mercy, and He will admit the people of the Fire into the Fire. Then He will say, ‘Bring out whoever you find with faith in his heart equal to a mustard seed.’ They will be brought out of it, scorched and turned to charcoal. They will be cast into the River of Life—or the River of Growth—and will grow there as a seed grows beside a torrent. Have you not seen how it emerges yellow and curled?”