Sahih Bukhari Volume 003, Book 031, Hadith Number 153.

Narated By Abu Bakr bin 'Abdur-Rahman : My father and I went to 'Aisha and she said, "I testify that Allah's Apostle at times used to get up in the morning in a state of Janaba from sexual intercourse, not from a wet dream and then he would fast that day." Then he went to Um Salama and she also narrated a similar thing.