Musnad Ahmad 25:226

Waki’ narrated to us, Usamah ibn Zayd narrated to us, he said: I asked Tawus about the voluntary prayer (al-subhah) during travel, and al-Hasan ibn Muslim ibn Yanaq was sitting there. Al-Hasan ibn Muslim said, while Tawus was listening: Tawus narrated to us, from Ibn Abbas, he said: "The Messenger of Allah, peace and blessings of Allah be upon him, ordained the prayer of residence and travel; so just as you pray in residence before it and after it, pray in travel before it and after it." Waki’ said once: "And perform it in travel."