Musnad Ahmad 25:716

Abdullah said: I found this hadith in my father’s book: Hajjaj narrated to us, Shu’bah narrated to us, from Abu Ishaq, from Abu al-Safar, from Sa’id ibn Shafi’, from Ibn Abbas: That they began asking him about prayer while traveling. Ibn Abbas said: "When the Prophet, peace and blessings of Allah be upon him, would depart from his family, he would not exceed two rak'ahs until he returned."