Muwatta Malik, Book 18, Hadith 25 (USC-MSA)

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used not to fast while travelling.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ لاَ يَصُومُ فِي السَّفَرِ ‏.‏

sunnah.com

also referenced under: Book 18, Hadith 659 (In-Book)