Sahih Muslim | Book 10 | Hadith Number 3656

Narrated by Ibn 'Umar (Allah be pleased with them)
Ibn 'Umar (Allah be pleased with them) reported Allah's Messenger (may peace be upon him) as saying: Both parties in a business transaction have the right to annul it so long as they have not separated; except in transactions which have been made subject to the right of parties to annul them.