Musnad Ahmad 31:801

Yunus narrated to us, Abu 'Awanah narrated to us, from Mansur ibn Zadhan, from al-Walid Abu Bishr, from Abu al-Siddiq, from Abu Sa'id al-Khudri, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to stand in the Dhuhr prayer during the first two rak'ahs, reciting in each rak'ah the equivalent of thirty verses. In the last two, he would recite in each rak'ah the equivalent of fifteen verses. He used to stand in the 'Asr prayer during the first two rak'ahs, reciting in each rak'ah the equivalent of fifteen verses, and in the last two, the equivalent of half of that."