He had intercourse with his wife when she was menstruating and was unaware of the shar’i ruling
Question

                I had intercourse with my wife when she was menstruating and I did not know what the ruling on having intercourse with a menstruating woman was. Please advise us, may Allaah reward you with good.
Praise be to Allah.Ignorance of the shar’i 
    ruling is a valid excuse before Allaah, may He be glorified and exalted. 
    Allaah has shown mercy to this ummah and lightened its burden, as He has 
    excused them from sin in cases of ignorance, forgetfulness and compulsion. 
It was narrated that Ibn 
    ‘Abbaas (may Allaah be pleased with him) said: When this verse was revealed 
    – “and whether you disclose what is in your own selves or conceal it, 
    Allaah will call you to account for it” [al-Baqarah 2:284] – there 
    entered their hearts something that had never entered them before. The 
    Prophet  (peace and blessings of Allaah be upon him) said: “Say: We hear 
    and we obey and we submit.” Then Allaah put faith in their hearts and Allaah 
    revealed the words: “Allaah burdens not a person beyond his scope. He 
    gets reward for that (good) which he has earned, and he is punished for that 
    (evil) which he has earned. ‘Our Lord! Punish us not if we forget or fall 
    into error”, Allaah said: I have granted that. “Our Lord! Lay not on 
    us a burden like that which You did lay on those before us (Jews and 
    Christians)” Allaah said: I have granted that. “Pardon us and grant us 
    forgiveness. Have mercy on us. You are our Mawlaa (Patron, Supporter and 
    Protector)” Allaah said: I have granted that. [al-Baqarah 2:286] 
    Narrated by Muslim (126). 
The scholars have stated 
    that the one who does a haraam deed out of ignorance is not to be blamed. 
Al-Nawawi said in Sharh 
    Muslim (3/204): 
If he forgot or was unaware 
    that there was menses, or he was unaware that it was haraam or he was forced 
    to do it, there is no sin on him and he does not have to offer expiation. 
    But if he had intercourse with her deliberately, knowing that she was 
    menstruating and that it was haraam, and did so willingly, then he has 
    committed a serious act of disobedience which al-Shaafa’i described as a 
    major sin, and he has to repent. End quote. 
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) said in al-Sharh al-Mumti’ (1/571): 
Expiation is not required 
    of the one who has intercourse with his wife when she is menstruating except 
    when three conditions are met: 

    1-That he was aware (of the 
    prohibition)

    2-That he was aware of what he 
    was doing

    3-That he did it willingly.
If he was unaware that it 
    is haraam or that she was menstruating, or he forgot, or the woman was 
    forced, or the menses began during intercourse, then he does not have to 
    offer expiation and there is no sin on him. End quote. 
Since you did not know of 
    the prohibition on having intercourse with a menstruating woman, there is no 
    sin on you in sha Allaah, and you do not have to offer expiation. 
But what every Muslim must 
    do is strive to understand Islam and learn the rulings, morals and etiquette 
    of sharee’ah, especially those that he needs for matters of worship, the 
    rulings on marriage and buying and selling, and what he needs in his 
    everyday life. He should not wait until he is about to do something and then 
    go and ask or use the excuse of ignorance and lack of knowledge. There is 
    the fear that the one who falls short in seeking essential Islamic knowledge 
    may be sinning. The Prophet  (peace and blessings of Allaah be upon him) 
    said: “Seeking knowledge is a duty upon every Muslim.” Narrated by Ibn 
    Maajah (224); classed as saheeh by al-Albaani in Saheeh Ibn Maajah.  
And Allaah knows best.