Reference: Fataawa Ramadhaan – Volume 2, Page 612, Fatwa No.607
Fataawa wa Rasaa.il Muhammad ibn Ibraaheem Aal ash-Shaykh – Volume 4, Page 195
Question: Regarding a man who had sexual intercourse with his wife on the first day of Ramadhaan, but did not know that was the first day of Ramadhaan?
Response: All Praise is due to Allaah [alone].
According to the math.hab [of Imaam Ahmad] he is required to make up [the fast] in addition to making an expiation. There is, however, another opinion [regarding this issue] and that is he is not required to make an expiation since he is excused; This is the opinion of Shaykh Taqi.uddeen [Ahmad Ibn Taymiyyah] and other than him, and this is [the] correct [opinion], in shaa.-Allaah.