Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2452
'A'isha, the wife of the Apostle of Allah (may peace be upon him), said: The dawn broke upon the Messenger of Allah (may peace be upon him) during the Ramadan in a state of junub not because of sexual dream (but on account of intercourse) and he washed himself and observed fast.