The Book of Prayer - Travellers
وَحَدَّثَنَاهُ يَحْيَى بْنُ حَبِيبٍ الْحَارِثِيُّ، حَدَّثَنَا خَالِدُ بْنُ الْحَارِثِ، ح وَحَدَّثَنَا عُبَيْدُ، اللَّهِ بْنُ مُعَاذٍ حَدَّثَنَا أَبِي قَالاَ، حَدَّثَنَا شُعْبَةُ، بِهَذَا الإِسْنَادِ نَحْوَهُ وَفِي حَدِيثِ خَالِدِ بْنِ الْحَارِثِ قَالَ عَلَى رَاحِلَةٍ يَسِيرُ وَهُوَ يَقْرَأُ سُورَةَ الْفَتْحِ .This hadlth has been narrated by Khalid al-Harith with the same chain of transmitters (with these words):(The Holy Prophet) was reciting Surat al-Fath as he was travelling on his mount
From Sahih Muslim, whose reports the scholars accept as authentic.