Sunan an-Nasa'i 2292
It was narrated that Al-'Awwam bin Hawshab said:
"I said to Mujahid: 'What about fasting while traveling?' He said: 'The Messenger of Allah used to fast (sometimes) and not fast (sometimes)."'
أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الْعَوَّامِ بْنِ حَوْشَبٍ، قَالَ قُلْتُ لِمُجَاهِدٍ الصَّوْمُ فِي السَّفَرِ قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَصُومُ وَيُفْطِرُ ‏.‏