Sahih Muslim 1111 c
The Book of Fasting
The strict prohibition of intercourse during the day in Ramadan for one who is fasting; And the obligation of offering major expiation and the definition thereof; And that it is obligatory for both the one who can afford it and the one who cannot afford it, and it remains an obligation for the one who cannot afford it until he has the means
Abu Huraira reported that a person had intercourse with his wife during Ramadan (while fasting). He asked for the religious verdict (about it) from the Messenger of Allah (ﷺ), whereupon he (the Prophet) said: Can you find a slave (to grant him freedom)? He said: No. He (the Prophet (ﷺ) again) said: Can you afford to observe fasts for two (consecutive) months? He said: No. He (the Prophet) said: Then feed sixty poor men.