What is required regarding the Takbir (Allahu Akbar) before Ruku (bowing)?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
The Takbir for entering Ruku must be uttered while standing, if standing is obligatory. If the worshiper begins to bow to the point of being in Ruku before completing the Takbir, the prayer is nullified, unless the prayer is a voluntary (Nafl) one, because standing is waived in voluntary prayers. The basis for this ruling is that the description of Ruku is distinct from the description of Qunut (if applicable or contextually referring to the standing posture before Ruku), and the Takbir must be completed while either standing or sitting, depending on the valid posture.
Supporting text
There is a differing probability that the prayer is nullified even if voluntary, because the worshiper did not perform the Takbir while standing, nor while sitting, which are the valid positions for commencing the act.