In The Name of Allaah, The Most Merciful, The Bestower of Mercy

The performer of the prayer says ‘Allaahu Akbar’ whilst standing based on his [or her] ability to do so [i.e. must stand whilst making the first Takbeer unless illness or injury prevents him or her]. Nothing else is permissible [to be said or uttered before the first Takbeer]. And the wisdom behind starting the [prayer with the statement Allaahu Akbar] is so that the performer of the prayer is reminded of the greatest of the One [i.e. Allaah] in whose presence he [or she] is standing and thus it brings about khas’yah [i.e. fear of Allaah based on having knowledge of Allaah’s greatness, absolute dominion, sovereignty etc].
Shaikh Abdul Muhsin Al-Abbaad [hafidhahullaah] said:
So, the performer of the prayer begins the prayer with the statement ‘Allaahu Akbar’ and this is called the ‘Takbeeratul Ih’raam’. It is given this name because after saying it, everything that was permissible for the performer of the prayer before he or she made the ‘Takbeeratul Ih’raam’ becomes impermissible, such as talking, eating, drinking and other than it. This is because the Messenger [sallal laahu alayhi wasallam] said:
The key to the prayer is purification [i.e. one must fulfil all the requirements of physical purification before the prayer], Wa Tahreemuhaa At-Takbeer [i.e. after one says Allaah Akbar, other actions such as eating, talking, drinking become impermissible], Wa Tahleeluhaa At-tasleem [i.e. all other actions, such as eating, drinking, talking becomes permissible after the Tasleem].