Musnad Abu Ya'la al-Mawsili 42:319

Zuhayr narrated to us, Hushaym narrated to us, from Mansur, from al-Walid ibn Muslim, from Abu al-Siddiq, from Abu Sa’id, 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. We estimated his standing 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 "Alif Lam Mim Tanzil al-Sajdah." In the last two rak’ahs, it was half of that. We estimated his standing in the first two rak’ahs of the ‘Asr to be the length of the last two of the Dhuhr, and the last two of the ‘Asr to be half of that.