Bulugh al-Maram #783
Fasting
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَأَصْلُهُ فِي
" اَلْمُتَّفَقِِ " مِنْ حَدِيثِ عَائِشَةَ; { أَنَّ حَمْزَةَ بْنَ عَمْرٍو سَأَلَ } 11 - صحيح. رواه البخاري ( 4 / 179 / فتح )، ومسلم ( 2 / 789 ) وتمامه: رسول الله صلى الله عليه وسلم عن الصيام في السفر، فقال: " إن شئت فصم، وإن شئت فافطر ".
And it's basic meaning is in Al-Bukhari and Muslim from the narration of Aisha (with the wording) "that Hamza bin 'Amro asked".
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About Bulugh al-Maram
This hadith is #783 from Bulugh al-Maram, in the Book of Fasting.