Sahih Bukhari
Fasting
Volume: 003 / Book Number: 031 / Hadith Number: 153
Narrated 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.