The Proof of Ijtihad اِجتِھاد and Qiyas قیاس
Finally, let us now refer to the statement: فَإِن تَنَازَعْتُمْ فِي شَيْءٍ فَرُدُّوهُ إِلَى اللَّـهِ وَالرَّسُولِ in Verse An-Nisa 4:59 ⧉ (Then, if you quarrel about something, revert it back to Allah and the Messenger.)
This 'reverting back to Allah and the Messenger' takes two forms:
(1) Revert to ahkam, the code of commands as mandated in the Book of Allah and the Sunnah of the Messenger of Allah.
(2) If the textually-mandated ahkam do not exist, the act of reverting back will be accomplished by analogical deduction (Qiyas) as based on their precedents.
The words of the expression: فَرُدُّہ (revert it back) are general, thus being inclusive of both forms.