Musnad Ahmad 5:539

Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to me, Abu Bakr ibn Ayyash narrated to us, from Abu Ishaq, from Hubayrah, from Ali, who said: "When the ten [days of Ramadan] entered, the Messenger of Allah (peace and blessings of Allah be upon him) would wake his family and tighten his waist-wrapper." It was said to Abu Bakr: "What is 'tightening the waist-wrapper'?" He said: "He would abstain from [sexual relations with] women."