Musnad Ahmad 1153:381

Aswad narrated to us, Sharik narrated to us, from Jabir, from Yazid ibn Murrah, from Lamis, from Aisha, who said: "The Prophet, peace and blessings of Allah be upon him, used to mix the first twenty [nights of Ramadan] with sleep and prayer. But when the [last] ten entered, he would exert himself and tighten his waist-wrapper."