Bukhari Collection: ::Book Number 3:: Hadith # 766
Narrated Aisha:Whenever Allah's Apostle wanted to go on a journey, he would draw lots as to which of his wives would accompany him. He would take her whose name came out. He used to fix for each of them a day and a night. But Sauda bint Zam`a gave up her (turn) day and night to `Aisha, the wife of the Prophet in order to seek the pleasure of Allah's Apostle (by that action).