Musnad Ahmad 25:321

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Abu Ishaq, from Abu al-Safar, from Sa'id ibn Shafi', from Ibn Abbas, who said: People began asking him about prayer while traveling. He said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would depart from his family, he would not pray except two rak'ahs until he returned to his family."