وَأَنْ تَكُونَ السُّورَةُ الْمَضْمُومَةُ لِلْفَاتِحَةِ مِنْ طِوَالِ الْمُفَصَّلِ فِي الْفَجْرِ وَالظُّهْرِ، وَمِنْ أَوْسَاطِهِ فِي الْعَصْرِ وَالْعِشَاءِ، وَمِنْ قِصَارِهِ فِي الْمَغْرِبِ لِلْمُقِيمِ ... وَإِطَالَةُ الْأُولَى فِي الْفَجْرِ فَقَطْ
For a resident, the commentary describes the Mufassal ranges approximately as:
• longer: from al-Hujurat toward al-Buruj — Fajr and Zuhr;
• middle: from around al-Buruj toward al-Bayyinah — Asr and Isha;
• shorter: from al-Bayyinah to the end of the Quran — Maghrib.
A traveler may choose any convenient Surah. The author also lists making the first rak'ah longer than the second specifically in Fajr.