Sayyidah Aishah (may Allah be pleased with her) narrated: When the Prophet (peace and blessings be upon him) travelled, he would sometimes offer the complete prayer and sometimes shorten it.
Imam al-Bazzar recorded it; its chain contains a narrator, al-Mughirah bin Ziyad, about whom there is disagreement regarding using him as evidence.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2952