Sahih Muslim 906bBook 10, Hadith 16
وَحَدَّثَنِي سَعِيدُ بْنُ يَحْيَى الأُمَوِيُّ، حَدَّثَنِي أَبِي، حَدَّثَنَا ابْنُ جُرَيْجٍ، بِهَذَا الإِسْنَادِ مِثْلَهُ وَقَالَ قِيَامًا طَوِيلاً يَقُومُ ثُمَّ يَرْكَعُ وَزَادَ فَجَعَلْتُ أَنْظُرُ إِلَى الْمَرْأَةِ أَسَنَّ مِنِّي وَإِلَى الأُخْرَى هِيَ أَسْقَمُ مِنِّي .
" It was for a long duration that he (the Holy Prophet) observed qiyam and he would then observe ruku'. (The narrator also added) I (Asma') looked at a woman who was older than I, and at another who was weaker than I.