Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with the same chain of narration (it is narrated from Sayyiduna Ibn Umar, may Allah be pleased with them both) that the Messenger of Allah, peace and blessings be upon him, said: "The buyer and the seller both have the right of option over their companion until they separate, except if the transaction is one in which the right of option remains even after separation."