From Sahih Bukhari (Hadith 6821):
Narrated Abu Huraira: A person had sexual relation with his wife in the month of Ramadan (while he was fasting), and he came to Allah's Messenger (ﷺ) seeking his verdict concerning that action. The Prophet (ﷺ) said (to him), "Can you afford to manumit a slave?" The man said, "No." The Prophet (ﷺ) said, "Can you fast for two successive months?" He said, "No." The Prophet (ﷺ) said, "Then feed sixty poor persons
Reference: Bukhari 6821