Is a prayer valid if performed without facing the qibla?

If a person knowingly prays facing a direction other than the qibla, the prayer is invalid. However, if someone does not know the direction of the qibla, they should make an effort to find it and pray according to the information or conviction they acquire. If they realize after completing the prayer that they made a mistake, the prayer is still valid and does not need to be repeated. If, after making an effort, they realize during the prayer that they are facing the wrong direction, they should turn to the correct direction within the prayer and continue. If someone prays in a random direction without making any effort or asking anyone, and it turns out that the direction was not the qibla, they must repeat the prayer. (Ibn Abidin, Radd al-Muhtar, 1/435)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu