Musnad Abu Ya'la al-Mawsili 42:153

Narrated to us Ishaq, narrated to us Hushaym, narrated to us Mansur ibn Zadhan, 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 thirty verses, each rak’ah being the length of the recitation of *Tanzil al-Sajdah* (Surah as-Sajdah). We estimated his standing in the first two rak’ahs of the ‘Asr to be the length of the last two [rak’ahs] of the Dhuhr. And we estimated his standing—meaning in the last two [rak’ahs]—to be half of that.