Narrated 'Abdullah:
While Allah's Apostle was prostrating (as stated below).
Vol: 001 Book: 004 Hadith: 240
A Selected Hadith
Narrated Ibn 'Umar:
The Prophet said, "The buyer and the seller have the option to cancel or confirm the bargain before they separate from each other or if the sale is optional." Nafi said, "Ibn 'Umar used to separate...
O Allah, Your male slave and the child of Your female slave is in need of Your mercy, and You are not in need of his torment. If he was pious then increase his rewards and if he was a transgressor then pardon him.
Info: Al-Hakim 1/359 who graded it authentic and Ath-Thahabi agreed with him. See also Al-Albani, Ahkamul-Jana'iz, p. 125.