Muwatta Imam Malik Book of Fasting, Hadith 24

Yahya related to me from Malik from Hisham ibn Urwa from his father that Hamza ibn Amr al-Aslami once said to the Messenger of Allah ﷺ "Messenger of Allah, I am a man who fasts. Can I fast when travelling?" The Messenger of Allah ﷺ said, "If you want you can fast, and if you want you can break the fast."

وَحَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّ حَمْزَةَ بْنَ عَمْرٍو الأَسْلَمِيَّ، قَالَ لِرَسُولِ اللَّهِ ﷺ يَا رَسُولَ اللَّهِ إِنِّي رَجُلٌ أَصُومُ أَفَأَصُومُ فِي السَّفَرِ فَقَالَ لَهُ رَسُولُ اللَّهِ ﷺ " إِنْ شِئْتَ فَصُمْ وَإِنْ شِئْتَ فَأَفْطِرْ " .