What is the motivation behind having different numbers for different prayers? Is there any significance attached to it, as in one prayer is different than others? Is this Allah's word (I doubt for I don't think its mentioned in Quran) or Sunnah? What are some credible/authoritative sources to know more about this?
Prayer is the direct command of Allah conveyed to the Prophet (without intermediaries Jibril) at Isra 'Mi'raj. As for the number of rakaat in the sholah is delivered Prophet Muhammad and taught by the Prophet SAW
"Pray ye as ye see me pray"
So we just have to listen and obey