The Chapter of the Qibla
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 43
وَ سَأَلَهُ مُعَاوِيَةُ بْنُ عَمَّارٍ: عَنِ اَلرَّجُلِ يَقُومُ فِي اَلصَّلاَةِ ثُمَّ يَنْظُرُ بَعْدَ مَا فَرَغَ فَيَرَى أَنَّهُ قَدِ اِنْحَرَفَ عَنِ اَلْقِبْلَةِ يَمِيناً أَوْ شِمَالاً فَقَالَ لَهُ «قَدْ مَضَتْ صَلاَتُهُ وَ مَا بَيْنَ اَلْمَشْرِقِ وَ اَلْمَغْرِبِ قِبْلَةٌ.
Muawiya ibn Ammar asked Him about a man who stands for prayer and, after finishing, realizes that he had deviated to the right or the left of the Qiblah. Imam عليه السلام replied: "His prayer is valid, for the area between the east and the west is (considered) the Qiblah."
- Ṣaḥīḥ al-Faqīh, p. 42