Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.120.

Ali reported that the Prophet, upon whom be peace, said, “The key to prayer is purity. What puts one into its inviolable state is the takbir, and the tasleem releases one from it.”

As to the authenticity of the report, it is related by ash-Shai’i, Ahmad, Abu Dawud, Ibn Majah and at-Tirmizhi, who called it the most authentic report on this topic. Al-Hakim and Ibn as-Sakin consider it as sahih. The takbir consists of saying Allahu akbar. Abu Hameed reported that when the Prophet stood for prayer, he would stand straight, raise his hands and say, “Allahu akbar.”

This is related by Ibn Majah, and in the Sahihs of Ibn Khuzaimah and Ibn Hibban. Al-Bazzar related something similar to it, but with a chain that is sahih according to Muslim’s criterion. ‘Ali and others also reported this.