Translation
(66) Virtues of the Qur'an
كتاب فضائل القرآن
(30) Chapter: At-Tarji’
(30) باب التَّرْجِيعِ
Sahih al-Bukhari 5047 

Narrated `Abdullah bin Mughaffal:

I saw the Prophet (ﷺ) reciting (Qur'an) while he was riding on his she camel or camel which was moving, carrying him. He was reciting Surat Fath or part of Surat Fath very softly and in an Attractive vibrating tone.

حَدَّثَنَا آدَمُ بْنُ أَبِي إِيَاسٍ، حَدَّثَنَا شُعْبَةُ، حَدَّثَنَا أَبُو إِيَاسٍ، قَالَ سَمِعْتُ عَبْدَ اللَّهِ بْنَ مُغَفَّلٍ، قَالَ رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم يَقْرَأُ وَهْوَ عَلَى نَاقَتِهِ ـ أَوْ جَمَلِهِ ـ وَهْىَ تَسِيرُ بِهِ وَهْوَ يَقْرَأُ سُورَةَ الْفَتْحِ أَوْ مِنْ سُورَةِ الْفَتْحِ قِرَاءَةً لَيِّنَةً يَقْرَأُ وَهْوَ يُرَجِّعُ‏.‏
Reference : Sahih al-Bukhari 5047
In-book reference : Book 66, Hadith 71
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 567
  (deprecated numbering scheme)