They prayed facing a direction other than the qiblah for a long time

Dear Brothers & Sisters,
As-Salaamu-Alaikum wa Rahmatullahi wa Barakatuh. (May Allah's Peace, Mercy and Blessings be upon all of you)
One of our brothers/sisters has asked this question:
A group of women rented a flat and they asked about the qiblah, then they prayed facing the direction that they were told about. Then they found out that it was wrong. What is the ruling?.
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Check below answers in case you are looking for other related questions:
He prayed then he heard the muezzin – should he repeat the prayer?
He joined the prayer behind the imaam but his intention was different from that of the imaam

Answer:
Praise be to Allaah.

Firstly: 

If it was slightly off, then it does not matter. 

Secondly: 

If a person tries his best to find out the qiblah, whether by asking local people who appear to him to be sincere, or otherwise, then he has done what is required of him. If he finds out later on that it was wrong, his prayer is still valid and he does not have to repeat it.

 It says in al-Mughni by Ibn Qudaamah (may Allaah have mercy on him): 

Whoever prays facing a certain direction after striving to work it out, then he finds out that he got the qiblah wrong, does not have to repeat it. In other words, if a person does his best to work out the direction and prays, then he finds out for certain that he prayed in a direction other than the direction of the Ka’bah, he does not have to repeat it, and the same applies to the one who follows him. This is the view of Maalik, Abu Haneefah and al-Shaafa’i, according to one of his two opinions.

Whatever written of Truth and benefit is only due to Allah's Assistance and Guidance, and whatever of error is of me. Allah Alone Knows Best and He is the Only Source of Strength.
Your brother and well wisher in Islam

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