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."' (TR)
English reference (TR): Vol. 3, Book 12, Hadith 2185 (TR)
Arabic reference (TR): Book 12, Hadith 2269 (TR)