Mishkat al-Masabih 7 Fasting كتاب الصوم 63

وَعَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: إِنَّ حَمْزَةَ بْنَ عَمْرٍو الْأَسْلَمِيَّ قَالَ لِلنَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَصُومُ فِي السَّفَرِ وَكَانَ كَثِيرَ الصِّيَامِ. فَقَالَ: «إِنْ شِئْتَ فَصم وَإِن شِئْت فَأفْطر»

‘A’isha said that Hamza b. ‘Amr al-Aslami who was greatly devoted to fasting asked the Prophet whether he should fast when on a journey, and received the reply, “Fast if you like, or break your fast if you like.” (Bukhari and Muslim.)