From Sahih Bukhari (Hadith 4507):
Narrated Salama:When the Divine Revelation: "For those who can fast, they had a choice to either fast, or feed a poor for every day," (2.184) was revealed, it was permissible for one to give a ransom and give up fasting, till the Verse succeeding it was revealed and abrogated it
Reference: Bukhari 4507