Description of the Prayer from Its Beginning to Its End
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 46
وَ سَأَلَ أَبُو بَصِيرٍ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنْ عِلَّةِ اَلصَّلاَةِ كَيْفَ صَارَتْ رَكْعَتَيْنِ وَ أَرْبَعَ سَجَدَاتٍ قَالَ «لِأَنَّ رَكْعَةً مِنْ قِيَامٍ بِرَكْعَتَيْنِ مِنْ جُلُوسٍ.
Abu Basir asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about the reason for why the prayer consists of two units (rak'ahs) with four prostrations (Sujood). Imam عليه السلام replied: "Because one unit of standing (qiyam) is equivalent to two units of sitting."
- Ṣaḥīḥ al-Faqīh