It was narrated from Dawud bin Salih Al Madani that his father said:
I heard Abu Sa'eed Al-Khudri say: "The Messenger of Allah (ﷺ) said: 'Transactions may only be done by mutual consent."' (DE)
English reference (DE): Vol. 3, Book 12, Hadith 2185 (DE)
Arabic reference (DE): Book 12, Hadith 2269 (DE)