Sunan an-Nasa'i, Volume 3, Book 22, Hadith 2306 (DarusSalam) ⓘ
It was narrated from Hamzah bin 'Amr Al-Aslami that he asked the Messenger of Allah: "Should I fast while traveling?" He said: "If you wish then fast, and if you wish then do not fast."
أَخْبَرَنَا مُحَمَّدُ بْنُ إِسْمَاعِيلَ بْنِ إِبْرَاهِيمَ، عَنْ مُحَمَّدِ بْنِ بِشْرٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ حَمْزَةَ بْنِ عَمْرٍو الأَسْلَمِيِّ، أَنَّهُ سَأَلَ رَسُولَ اللَّهِ صلى الله عليه وسلم أَصُومُ فِي السَّفَرِ قَالَ " إِنْ شِئْتَ فَصُمْ وَإِنْ شِئْتَ فَأَفْطِرْ " .
also referenced under: Hadith 2304 (Reference) & Book 22, Hadith 215 (In-Book)