‘Abdullaah bin Ahmad bin Hanbal said: I asked my father concerning a man who has intercourse with his wife in the month of Ramadhaan thinking that he is still in the night period whereas he has actually entered the morning time.
So he said:
((He has to make up that fast, so he makes up for a day and he should seek expiation* of it in accordance to that which the Prophet (Sallalaahu ‘alaihi wa sallam) commanded the one who has intercourse with his wife in the month of Ramadhaan.))
*Shaikh ul Islaam Ibn Taymiyyah said:
Concerning this issue then there are three statements from the people of knowledge in relation to it;
The first of them: That he has a fast to make up for – as well as seeking its expiation, and this is something well known from the madhab of Ahmad.
The second of them: That he has a fast to make up for - and this is a second saying from the Madhab of Ahmad.
The third of them: That he has neither a fast to make up for, nor does he seek its expiation, and this is a saying from some groups of the Salaf … and this statement is the most correct of statements as well as the most suited to the Usool of the Sharee’ah and the proofs of The Book and the Sunnah, and it is analogy of the Usool of Ahmad and other than him. (See Majmoo’ al-Fataawaa vol 25 p.264)