Musnad Ahmad 938:5

Abu Ahmad narrated to us, Kathir ibn Zayd narrated to us, from al-Muttalib ibn Abdullah, who said: They disputed regarding the recitation in the Dhuhr and Asr prayers, so they sent to Kharijah ibn Zayd. He said: My father said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to stand, or he would prolong the standing, and he would move his lips." "And he (the father) knew that this was not for anything other than recitation, so I do that."