931 - 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.
Hadith.931 - Abu Basir asked Imam Jafar ibn Muhammad Al-Sadiq 
about the reason for why the prayer consists of two units (rak'ahs) with four prostrations (Sujood).
Imam replied: "Because one unit of standing (qiyam) is equivalent to two units of sitting."
