Is it obligatory to repeat the prayer if the worshiper did not know that ritual bathing (ghusl) from major ritual impurity (janabah) was required before performing it?

A Muslim must learn the religious rulings, especially those related to their duties, such as the rulings on Zakat, trade, purification, and prayer.

Regarding the questioner's unawareness of the obligation of ghusl (ritual bath) after janabah (sexual defilement) and his prayer in that state, scholars have held that this is considered an excuse. Therefore, he is not obliged to make up past prayers. However, he must perform ghusl and repeat the present prayer, the ruling for which became known at its time.

They based this on the hadith of the man who erred in his prayer, whom the Prophet (peace be upon him) commanded to repeat only the present prayer. They also cited the story of Umar and Ammar (may Allah be pleased with them both) when they were ignorant of the rulings of tayammum (dry ablution), and the Prophet (peace be upon him) did not command them to make up the missed prayers.

Shaykh al-Islam Ibn Taymiyyah clarified that the correct view in these matters is that repetition is not obligatory because Allah has forgiven error and forgetfulness, and He would not punish anyone before sending messengers.

It can be said that if someone was in a place where means of learning were available and he neglected them, then he must make up the prayers he performed without ghusl, unless he is very old, in which case the making up is waived due to hardship.