Chapter on the Prayer Times (Mawaqit)
Tahdhīb al-Aḥkām|Volume 2|Book 1|Chapter 13
وَ عَنْهُ عَنِ اَلْمِيثَمِيِّ عَنْ أَبَانٍ عَنْ إِسْمَاعِيلَ اَلْجُعْفِيِّ عَنْ أَبِي جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ قَالَ:« أَتَدْرِي لِمَ جُعِلَ اَلذِّرَاعُ وَ اَلذِّرَاعَانِ» قَالَ قُلْتُ لِمَ قَالَ« لِمَكَانِ اَلْفَرِيضَةِ لِئَلاَّ يُؤْخَذَ مِنْ وَقْتِ هَذِهِ وَ يُدْخَلَ فِي وَقْتِ هَذِهِ»
And from him, from Al-Mithami from Aban from Isma‘il Al-Ju‘fi:
Abu Ja‘far عليه السلام, peace be upon him, said: "Do you know why the arm's span (dhira‘) and the two arm's spans were set?" I asked: "Why?" Imam عليه السلام said: "Because of the obligatory prayer, so that time would not be taken from this (obligatory prayer) and entered into that (supererogatory prayer)."
- Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 2, p. 262
- Ṣaḥīḥ al-Tahdhīb, p. 158