Musnad Abu Ya'la al-Mawsili 155:310

From Abu Hurairah, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Fever and headache will not cease to afflict the believing man and woman, even if they have sins upon them like Uhud, until it leaves them while they have no more than the weight of a mustard seed of sins upon them."