Narrated Abu Said Al-Khudri:

The Prophet (ﷺ) said, "When the people of Paradise will enter Paradise and the people of Hell will go to Hell, Allah will order those who have had faith equal to the weight of a grain of mustard seed to be taken out from Hell So they will be taken out but (by then) they will be blackened (charred). Then they will be put in the river of Haya' (rain) or Hayat (life) (the Narrator is in doubt as to which is the right term), and they will revive like a grain that grows near the bank of a flood channel.

Reference : Sahih-Bukhari 22
In Book Reference : Book 2, Hadith 15