Congregational Prayer and Its Virtue
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 57
وَ قَدْ رُوِيَ: «أَنَّهُ إِنْ كَانَ فِي صَلاَةِ اَلظُّهْرِ جَعَلَ اَلْأَوَّلَتَيْنِ فَرِيضَةً وَ اَلْأَخِيرَتَيْنِ نَافِلَةً وَ إِنْ كَانَ فِي صَلاَةِ اَلْعَصْرِ جَعَلَ اَلْأَوَّلَتَيْنِ نَافِلَةً وَ اَلْأَخِيرَتَيْنِ فَرِيضَةً.
It has been narrated: "If it is during the Dhuhr prayer, he should make the first two rak'ahs obligatory and the last two rak'ahs voluntary. If it is during the Asr prayer, he should make the first two rak'ahs voluntary and the last two rak'ahs obligatory."
- Ṣaḥīḥ al-Faqīh