Jami` at-Tirmidhi, Book 47, Hadith 3217 (In-Book) ⓘ

Narrated Ibn 'Umar: "The Messenger of Allah would perform voluntary Salat upon his mount facing whichever direction he was headed, while he was coming from Makkah to Al-Madinah." Then Ibn 'Umar recited: To Allah belong both the east and the west. (2:115)" And Ibn 'Umar said: "It was about this that the Ayah was revealed."

حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا يَزِيدُ بْنُ هَارُونَ، أَخْبَرَنَا عَبْدُ الْمَلِكِ بْنُ أَبِي سُلَيْمَانَ، قَالَ سَمِعْتُ سَعِيدَ بْنَ جُبَيْرٍ، يُحَدِّثُ عَنِ ابْنِ عُمَرَ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّي عَلَى رَاحِلَتِهِ تَطَوُّعًا حَيْثُمَا تَوَجَّهَتْ بِهِ وَهُوَ جَاءٍ مِنْ مَكَّةَ إِلَى الْمَدِينَةِ ثُمَّ قَرَأَ ابْنُ عُمَرَ هَذِهِ الآيَةَ ‏:‏ ‏(‏وَلِلَّهِ الْمَشْرِقُ وَالْمَغْرِبُ ‏)‏ الآيَةَ ‏.‏ قَالَ ابْنُ عُمَرَ فَفِي هَذَا أُنْزِلَتْ هَذِهِ الآيَةُ ‏.‏ قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ ‏.‏