Ibn Majah

´It was narrated that Abu Hurairah said:` “A man came to the Prophet (ﷺ) and said: ‘I am doomed.’ He said: ‘Why are you doomed?’ He said: ‘I had intercourse with my wife in Ramadan.’ The Prophet (ﷺ) said: ‘Free a slave.’ He said: ‘I cannot.’ He said: ‘Fast for two consecutive months.’ He said: ‘I cannot.’ He said: ‘Feed sixty poor persons.’ He said: ‘I cannot.’ He said: ‘Sit down.’ So he sat down, and while doing so a basketful of dates was brought. The Prophet (ﷺ) said: ‘Go and give this in charity.’ He said: ‘O Messenger of Allah, by the One Who sent you with the truth, there is no household between its two lava fields (i.e., in Al-Madinah) that is more in need of it than us.’ He said: ‘Then go and feed your family.’” Another chain from Abu Hurairah with additional words: "Then he (the Prophet SAW) said: "And fast a day in its place."

With this chain also, Abu Huraira (may Allah be pleased with him) narrated from the Messenger of Allah (peace and blessings be upon him) in a similar manner. In it, it is mentioned that the Prophet (peace and blessings be upon him) said: "Fast one day in its place."

´It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:` “Whoever does not fast one day in Ramadan without having a concession allowing that, fasting for a lifetime will not make up for that.”