Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 250
Narated By 'Aisha, Ummul Mu'minin : The Apostle of Allah (pbuh) took a bath and offered two rak'ahs of prayer and said the dawn prayer. I do not think he performed ablution afresh after taking a bath.