بُلُوغُ الْمَرَامِ
Bulugh al-Maram 287
The Book of Prayer
وَعَنْ أَبِي سَعِيدٍ اَلْخُدْرِيِّ - رضى الله عنه - قَالَ : { كُنَّا نَحْزُرُ قِيَامَ رَسُولِ اَللَّهِ - صلى الله عليه وسلم -فِي اَلظُّهْرِ وَالْعَصْرِ , فَحَزَرْنَا قِيَامَهُ فِي اَلرَّكْعَتَيْنِ اَلْأُولَيَيْنِ مِنْ اَلظُّهْرِ قَدْرَ : (الم تَنْزِيلُ) اَلسَّجْدَةِ . وَفِي اَلْأُخْرَيَيْنِ قَدْرَ اَلنِّصْفِ مِنْ ذَلِكَ . وَفِي اَلْأُولَيَيْنِ مِنْ اَلْعَصْرِ عَلَى قَدْرِ اَلْأُخْرَيَيْنِ مِنْ اَلظُّهْرِ , وَالْأُخْرَيَيْنِ مِنْ اَلظُّهْرِ } رَوَاهُ مُسْلِمٌ . [1]
Narrated Abu Sa'id Al-Khudri (RA): We used to estimate the length of the standing position of Allah's Messenger (ﷺ) in the Zuhr and 'Asr prayers, and we estimated that he stood in the first two Rak'a of the Zuhr prayer as long as it takes to recite Alif-Lam-Mim, Tanzil (Surat As-Sajdah) and in the last two Rak'a, half the time of that. And in the first two Rak'a of 'Asr, he used to stand as long as the last two of the Zuhr. And the last two Rak'a of 'Asr used to be of about half the time of the first two." [Reported by Muslim).
Sunnah.com reference: Book 2, Hadith 173