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

Question

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.