Informed about correct direction of qiblah during prayer

If there is a congregation praying, and during their prayer they are informed about the
right direction of qiblah, they should all turn towards the correct direction. The same is
also true for someone praying individually. Whatever part of their prayer has been
performed (before changing direction) will be correct. The evidence for this a narration
by Imam Muslim from Anas (may Allah be please with him), that:

While the Prophet
(peace be upon him) was praying towards
Bayt-al-Maqdis (Jerusalem), the verse was revealed to him (translation of
meaning):

• Verily! We have seen the turning of your face towards
  the heaven. Surely, We shall turn you to a qiblah that
  shall please you, so turn your face in the direction of
  Al-Masjid Al-Haram.

  (Al-Baqarah, 2:144).

• A man from Bani Salamah was passing by and found
  them (i.e., the people of Bani Salamah) in the state of
  ruku‘ in the Fajr prayer in the second rak‘ah. He
  shouted to them: Indeed the qiblah has been changed.
  So, turn to the qiblah as they (i.e., the Prophet

  and his Companions) have.

  (Sahih Musim, No. 527)

 If some of the people were informed and the others were not, then the one to whom it
was made clear should turn to the direction he believes to be the correct direction of
qiblah. Now if all of these people (who were informed or not) were originally praying
together in the same direction, and some of them turned towards right and some
towards left, it is correct for some to follow others. But the scholars have a difference
of opinion about some people following others in the situation where there was
complete disagreement (among the praying people) about the direction of the qiblah. If
there was someone among them who was completely ignorant about the direction, he
should follow the one who is more aware amongst them of the direction of the qiblah.¹
For the one who was uninformed about the direction f the qiblah, must ask someone if
he can, or else he should make ijtihad (make a judgment based on the best of his ability
with the information available) if he is able to, otherwise he must follow someone who is
reliable. If he cannot find one, then he should do his best in obedience to Allah and
complete his prayer, and his prayer is correct. This sometimes happens to a number of
those who travel to the lands of disbelievers and find no Muslim around them to inform
them of the correct direction of qiblah.

If one was capable of finding out the direction of the qiblah, but was neglectful and
prayed without making all possible efforts, he should repeat his prayer because he was
careless.²

¹ Al-Mughni, 1/473.

²Al-Mughni, 1/490