Prophet Muhammad (Peace and blessings of Allah be upon him) said :
When the people of Jannah will enter Jannah and the people of Jahannam will go to Jahannam, Allah will command to Angels to take out people from Jahannam who have had Iman equal to the weight of a grain of mustard seed. So they will be taken out, but they will be blackened (charred by the fire of Hell). Then they will be put in the river and they will revive like a grain that grows near the bank of a flood channel. (Al-Bukhari)