Musannaf Ibn Abi Shayba 3:6296

Abu Bakr ibn Ayyash narrated to us, from Abu Ishaq, from Abu Hurayrah, from Ali, who said: When the last ten days [of Ramadan] would enter, the Messenger of Allah (may Allah bless him and grant him peace) would wake his family and tighten his waist-wrapper. It was said to Abu Bakr, "What is [meant by] tightening the waist-wrapper?" He said, "Abstaining from women."