Sahih Muslim 452 a
Abu Sa'id al-Khudri reported:
We used to estimate how long Allah's Messenger (ﷺ) stood in the noon and afternoon prayers, and we estimated hat he stood in the first two rak'ahs of the noon prayer as long as it takes to recite Alif Lam Mim, Tanzil, i. e. as-Sajda. We estimated that he stood half that time in the last two rak'ahs; that he stood in the first two of the afternoon as long as he did in the last two at noon; and in the last two of the afternoon prayer about half that time. 
Abu Bakr in his narration has made no mention of Alif Lam Mim, Tanzil, but said: As long as it takes to recite thirty verses.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ جَمِيعًا عَنْ هُشَيْمٍ، - قَالَ يَحْيَى أَخْبَرَنَا هُشَيْمٌ، - عَنْ مَنْصُورٍ، عَنِ الْوَلِيدِ بْنِ مُسْلِمٍ، عَنْ أَبِي الصِّدِّيقِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، قَالَ كُنَّا نَحْزِرُ قِيَامَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي الظُّهْرِ وَالْعَصْرِ فَحَزَرْنَا قِيَامَهُ فِي الرَّكْعَتَيْنِ الأُولَيَيْنِ مِنَ الظُّهْرِ قَدْرَ قِرَاءَةِ الم تَنْزِيلُ السَّجْدَةِ وَحَزَرْنَا قِيَامَهُ فِي الأُخْرَيَيْنِ قَدْرَ النِّصْفِ مِنْ ذَلِكَ وَحَزَرْنَا قِيَامَهُ فِي الرَّكْعَتَيْنِ الأُولَيَيْنِ مِنَ الْعَصْرِ عَلَى قَدْرِ قِيَامِهِ فِي الأُخْرَيَيْنِ مِنَ الظُّهْرِ وَفِي الأُخْرَيَيْنِ مِنَ الْعَصْرِ عَلَى النِّصْفِ مِنْ ذَلِكَ ‏.‏ وَلَمْ يَذْكُرْ أَبُو بَكْرٍ فِي رِوَايَتِهِ الم تَنْزِيلُ وَقَالَ قَدْرَ ثَلاَثِينَ آيَةً ‏.‏