Facing the Qiblah is one of the prerequisites for the validity of ones prayer.

Allah ordered us in the Qu’ran to face the Qiblah when we are ready to start our Prayers. In Surah Al-Baqarah ayah 144 & 150, Allah said what means: “then turn your face in the direction of the sacred mosque, and wherever you are, turn your faces in its direction.” From this ayah we can see that the order is to face in the DIRECTION of the Qiblah.
 
The Prophet (Sallallahu ^alayhi wasallam) pointed at the Ka^bah and said, “This is the Qiblah.” Narrated by Bukhariyy & Muslim.

There is a Islamic scholarly consensus that for ones prayer to be valid one must direct oneself to the Qiblah.

Ibn ^Abidin, who is among the famous Hanafiyy scholars said: “If he commenced (his prayers) without detecting (al-Qiblah), his prayer would not be valid.”

The person who sees the Ka^bah faces it in prayer. The one who does not see the Ka^bah takes the word of a trustworthy person who sees the Ka^bah. The one who is not near the Ka^bah makes effort to find the direction of the Qiblah using the signs that Allah created for us.

Ayah 16 of Surah An-Nahl means: “and marks and by the stars they guide themselves.” Ibn ^Abidin al-Hanafiyy in commenting on the following text of Ad-Durr al-Mukhtar: {Al-Qiblah would be seen and known by the sign which, in the villages and countries, is the maharib of the Companions and their followers; and in the open lands and seas, is the stars like Polaris (North Star).} said: “His saying [like Polaris], is the strongest of the signs.”

In his book, Tabyin al-Haqa’iq, Az-Zayla^iyy copied the saying of Abu Hanifah which, likewise, Khayrud-Din ar-Ramliyy reported in his book, Al-Fatawa al-Khayriyyah, that the Qiblah of the people of the western countries is toward the east; the Qiblah of the people of the eastern countries is toward the west; the Qiblah of the people of the southern countries is toward the north; and the Qiblah of the people of the northern countries is toward the south.

As it is well known, the Companions, may Allah reward them, had al-’ijma^ (unanimous agreement) to look at the position of the country in relation to Makkah when they wanted to place a mihrab in it. Hence, if the position of the country was north of Makkah, they directed the mihrab toward south, and if it was south of Makkah, the direct the mihrab toward north, and if the country was east, they direct the mihrab toward the west and vice-versa. These maharib still exist in the Muslim countries, and the sayings of the Companions are documented in the books of the scholars. The aforementioned hadith of the Prophet, and doings of the Companions signify that determining the direction of al-Qiblah relies on knowing the position of the country from Makkah based on the sun, moon, stars, and the like.

If one stands in Makkah, looking at the height of Polaris above the horizon and compares it to the height of Polaris above the horizon while standing in the USA or Canada, he would find the Polaris is higher above the horizon in the USA and Canada. This means that these countries fall north of Makkah. Therefore, to face al-Ka^bah it is a must to direct oneself southwards. We know that the sun rises in Makkah before North America; therefore we know that Makkah is east in relation to North America. Moreover, if you add that the USA and Canada are also westward of Makkah, which is at the center of earth, then it is necessary for he who wants to pray while in these countries to face eastward so that he would be facing al-Qiblah.

The actual Fatwa from al-Azhar that Qiblah to North America is East South East

Praise be to Allah, may Allah raise the rank of our master Muhammad, his family members and companions, and protect his nation from that which he fears for it. Thereafter;
 
The proof to the issue of al-Qiblah in America is the consensus of the scholars of Islam in that the people of the east face to west and the people of the north face the south and the people of the south face to north, due to that Makkah is at the center of the earth. Hence, the saying that the people of the North America should face the northeast is invalid, and rather the people of North America should face the southeast. He who deviates from that has deviated from the unanimous agreement and consensus of the scholars of ‘Islam, the khalaf and the Salaf. He who wishes, let him look in shuruhal Minhaj and the book of ‘Anwar ^Amal al-‘Abrar by al-Ardabiliyy and the book of Nihayat al-Matlab by Imam of al-Haramayn and others, among the books of ash-Shafi^iyy school. Also in al-Mabsut by as-Sarakhsiyy al-Hanafiyy among the early scholars and Ibn ^Abidin among the late scholars along with others among the scholars and knowledgeable persons of the four schools. And Allah, subhanahu wa ta^ala knows best.

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