It was narrated from Samurah that the Messenger of Allah (ﷺ), said:
"If two (separate) authorized persons make a sale (of the same thing), then the first transaction is the one that is valid. " (IT)
English reference (IT): Vol. 3, Book 12, Hadith 2191 (IT)
Arabic reference (IT): Book 12, Hadith 2275 (IT)