Sunan an-Nasa'i 4468
سنن النسائي#4468SahihIt was narrated from Ibn Juraij:"Nafi dictated to me, from Ibn 'Umar who said: The Messenger of Allah said: 'the two parties to a transaction both have the choice so long as they have not separated, unless they have both chosen to conclude the transaction. If they have both chosen to conclude the transaction, then the transactions binding." (Sahih)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 44:20