Musnad Ahmad 31:2

Hushaym narrated to us, Mansur—meaning Ibn Zadhan—narrated to us, from al-Walid ibn Muslim, from Abu al-Mutawakkil, or from Abu al-Siddiq, from Abu Sa’id al-Khudri, who said: "We used to estimate the standing of the Messenger of Allah, may Allah bless him and grant him peace, in the Dhuhr and the ‘Asr." He said: "We estimated the standing of the Messenger of Allah, may Allah bless him and grant him peace, in the Dhuhr in the first two rak'ahs to be the length of the recitation of thirty verses, the length of the recitation of Surah al-Tanzil al-Sajdah." He said: "And we estimated his standing in the last two to be half of that. And we estimated his standing in the ‘Asr in the first two rak'ahs to be half of that." He said: "And we estimated his standing in the last two to be half of the first two."