Yahya related to me from Malik from Nafi from Abdullah ibn Umarthat the Messenger of Allah, may Allah bless him and grant him peace,said, "Both parties in a business transaction have the right ofwithdrawal as long as they have not separated, except in thetransaction called khiyar."Malik said, "There is nospecified limit nor any matter which is applied in this case accordingto us."
USC-MSA web (English) reference: Book 31, Hadith 80
Arabic reference: Book 31, Hadith 1367