Sahih Muslim
The Book Pertaining to Repentance and Exhortation to Repentance (Kitab Al-Tauba)
Book Number: 037 / Hadith Number: 6657
This hadith has been narrated on the authority of Sulaiman Taimi with the same chain of transmitters that a person had taken liberty with a woman less than fomication. He came to 'Umar b. Khattab and he took it to be a serious offence. Then he came to Abu Bakr and he also took it to be a serious offence. Then he came the Allahs Apostle (may peace be upon him) and he made a mention of this to him. The rest of the hadith is the same.