Hadithcore
Sahih al-Bukhari, 4489
sahih

Narrated Anas:

None remains of those who prayed facing both Qiblas (that is, Jerusalem and Mecca) except myself.

حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا مُعْتَمِرٌ، عَنْ أَبِيهِ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ لَمْ يَبْقَ مِمَّنْ صَلَّى الْقِبْلَتَيْنِ غَيْرِي‏.‏

References2 variants
In-Book Reference
Book 65, Hadith 16
USC-MSA web (English) reference
Vol. 6, Book 60, Hadith 16 (deprecated numbering scheme)