(9) باب ذِكْرِ الاِخْتِلاَفِ عَلَى نَافِعٍ فِي لَفْظِ حَدِيثِهِ
it was narrated from Ismail, from Nafi, that Ibn 'Umar 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 they transaction. If they have both chosen to conclude the transaction, then the transaction is binding." (Sahih )
Reference: Sunan an-Nasa'i 4467
In-book reference: Book 44, Hadith 19
English translation: Vol. 5, Book 44, Hadith 4472