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

Yahya related to me from Malik from Hisham ibn Urwa from hisfather that Hamza ibn Amr al-Aslami once said to the Messenger ofAllah, may Allah bless him and grant him peace, "Messenger of Allah, Iam a man who fasts. Can I fast when travelling?" The Messenger ofAllah, may Allah bless him and grant him peace, said, "If you want youcan fast, and if you want you can break the fast." Translation Not Available

Book Ref: Muwatta Imam Malik Book 18 Hadith 658
Web Ref:  Muwatta Imam Malik Book 18 Hadith 24