Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrated: During travel the Prophet (peace and blessings be upon him) would sometimes fast and sometimes not fast, and I too (during travel) sometimes fast and sometimes do not fast.
Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir. It also has another chain, all of whose narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 4914