[Machine] The Messenger of Allah ﷺ used to perform nine units of voluntary prayer at night. When he grew older and became heavy, he reduced it to seven units. He would also perform two units of prayer while sitting, and in those two units, he would recite Surah Ar-Rahman (The Most Gracious) and Surah Al-Waqi'ah (The Inevitable). Anas (may Allah be pleased with him) said, "We used to recite the shorter Surahs when there was an earthquake, and we would recite Surah Al-Kafirun (The Disbelievers) and similar Surahs." Ammar ibn Zaid differed in reciting the Surahs that the Prophet ﷺ would recite in those two units, according to the other narrators.
كَانَ رَسُولُ اللهِ ﷺ يُوتِرُ بِتِسْعِ رَكَعَاتٍ فَلَمَّا أَسَنَّ وَثَقُلَ أَوْتَرَ بِسَبْعٍ وَصَلَّى رَكْعَتَيْنِ وَهُوَ جَالِسٌ فَقَرَأَ فِيهِمَا الرَّحْمَنَ وَالْوَاقِعَةَ قَالَ أَنَسٌ وَنَحْنُ نَقْرَأُ بِالسُّوَرِ الْقِصَارِ إِذَا زُلْزِلَتْ وَقُلْ يَا أَيُّهَا الْكَافِرُونَ وَنَحْوِهِمَا وَقَالَ مَرَّةً يَقْرَأُ فِيهِنَّ خَالَفَ عُمَارَةُ بْنُ زَاذَانَ فِي قِرَاءَةِ النَّبِيِّ ﷺ فِيهِمَا سَائِرَ الرُّوَاةِ