Musannaf Ibn Abi Shayba 3:1191

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Mansur, from Abu Bishr al-Hujaymi, 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 his standing in the Dhuhr in the first two rak'ahs to be the length of thirty verses, and we estimated his standing in the Dhuhr in the last two rak'ahs to be half of that. And 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 in the last two [rak'ahs] of the ‘Asr to be half of that.