Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.10676
Fataawa Ramadhaan – Volume 2, Page 615, Fatwa No.612
Question: A man had sexual intercourse with his wife during the [last part of the] night assuming the time of Fajr had not entered. However, after finishing, he left the [bed]room whereupon it became clear to him that he had sexual intercourse after the time of Fajr had entered. He was saddened and continued with his fast. However, he feels sinful and asks what is required [of him] as a result of his action [of sexual intercourse after the time of Fajr had began]?
Response: If the situation is as mentioned, then he is required to make up the fast for that day if it is clear to him that he had sexual intercourse after the obligation to abstain [from all that breaks the fast] by the entering of the time of Fajr. [In addition], he is required to free a believing (mu.min) slave and if he is unable to find one, then he is to fast two months consecutively. If he is unable to do so, he is to feed sixty poor and needy people as a result of his negligence in finding out about the beginning of the time of Fajr. The same is also required of the wife unless she was forced [into having sexual intercourse with her husband].
And with Allaah lies all success, and may Allaah send prayers and salutations upon our Prophet Muhammad ﷺ and his family and his companions.