Sunan Ibn Majah, Chapter: The Chapters on Dry Ablution

"I asked whether the Messenger of Allah (saw) prayed voluntary prayer when travelling, but I could not find anyone to tell me until Umm Hani' bint Abu Talib told me that he had come during the year of the Conquest (of Makkah). He ordered that a screen be held up, and that was done, and he took a bath; then he prayed eight Rak'ah (units) of voluntary prayer." (Sahih)