It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him): We used to estimate the standing of the Messenger of Allah (peace and blessings be upon him) in the Zuhr and Asr prayers. We estimated that in the first two rak'ahs of Zuhr he (peace and blessings be upon him) would stand for as long as it takes to recite "Alif Lam Mim. The revelation" [Surah al-Sajdah: 1-2] (Surah al-Sajdah). We estimated the last two rak'ahs at half of the first two, the first two rak'ahs of Asr as equal to the last two rak'ahs of Zuhr, and the last two rak'ahs of Asr at half of the first two rak'ahs of Asr.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4025