سنن النسائي

It was narrated that Abu Sa'eed Al-Khudri said: "We used to estimate how long the Messenger of Allah (ﷺ) stood when praying in Zuhr and 'Asr. We estimated that he stood in Zuhr for as long as it take to recite thirty verses, as long as Surat As-Sajadah in the fits two Rak'ahs, and half that in the last two. And we estimated that he stood for as long in the fits two Rak'ahs, and half that in the last two. And we estimated that he stood for as long in the first two Rak'ahs of 'Asr as he stood in the last two Rak'ahs of Zuhr, and we estimated that he stood half as long as that in the last two Rak'ahs of 'Asr."