Musnad Ahmad 936:5

Sufyan ibn 'Uyaynah narrated to us, from Isma'il ibn Umayyah, from the one who narrated to him, from the mother of the child of Ubayy ibn Ka'b, from Ubayy ibn Ka'b: That a man entered upon the Prophet, peace and blessings of Allah be upon him, and he said: "When was your last encounter with Umm Mildam?" — and it is the fever between the skin and the flesh. He said: "It is a pain that has never afflicted me." The Messenger of Allah, peace and blessings of Allah be upon him, said: "The likeness of the believer is like the tender plant; it turns red one time, and turns yellow another."