From Sahih Muslim (Hadith 2686):
Salama b. Akwa' reported:We, during the lifetime of the Messenger of Allah (ﷺ), in one month of Ramadan (observed fast according to our liking). He who wished to fast lasted and he who wished to break broke it and fed a needy person as an expiation 1544 till this verse was revealed:" He who witnesses among you the month (of Ramadan) he should observe fast during it" (ii)
Reference: Muslim 2686