Narrated Hakim b. Hizam:
That the Messenger of Allah (ﷺ) said: Both the buyer and the seller retain the option as long as they have not separated. If they spoke the truth and clarified (any defects or conditions), then they would be blessed in their sale, and if they hid something and lied then their sale would be deprived of blessings.
And this is a Sahih Hadith.
This is how it was reported from Abu Barzah Al-Aslami, that two men came disputing to him after the sale of a horse, and they were on a ship, so he said: I did not see the two of your separate, and the Messenger of Allah (ﷺ) said: 'The buyer and the seller retain the choice as long as they did not separate.'
Some of the people of knowledge, among the people of Al-Kufah and others, held the view that the separation refers to speech. This is the saying of [Sufyan] Ath-Thawri. This has been reported from Malik bin Anas, and it has been reported from Ibn al-Mubarak that he said: How could this be refuted ? And the Hadith about it from the Prophet (ﷺ) is Sahih, and it strenghtens this view.
And the meaning of the saying of the Prophet (ﷺ): Except for the optional sale is, that (while they are still together) the seller gives the buyer the option to cancel after the conclusion of the sale. If he chooses to agree to the sale, then he does not have the choice to cancel the sale after then, even if they did not separate. This is how Ash-Shafi'i and others explained it. And what strenghtens the view of those who said that the separation refers to them parting, (and) it does not refer to speech, is the (following) Hadith of 'Abdullah bin 'Amr from the Prophet (ﷺ).