Musnad Ahmad 5:550

Abdullah narrated to us, Abu Musa Muhammad ibn al-Muthanna narrated to me, Abu Bakr ibn Ayyash narrated to us, Abu Ishaq narrated to me, from Hubayrah ibn Yarim, from Ali, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to wake his family during the last ten [nights of Ramadan], and he would tighten his waist-wrapper."