The Messenger of Allah ﷺ said, "The key to prayer is purification, its sanctity begins with the takbir, and its conclusion is the taslim."
حدثنا عثمان بن أبي شيبة حدثنا وكيع عن سفيان عن ابن عقيل عن محمد ابن الحنفية
قال رسول الله ﷺ مفتاح الصلاة الطهور وتحريمها التكبير وتحليلها التسليم
[Hasan] Tirmidhi narrated it in the chapters on purification, the chapter on what has been said that the key to prayer is purification, no. 3, and Ibn Majah, no. 275, from the hadith of Waki with the same wording. Baghawi graded it hasan (Sharh al-Sunnah, no. 558). The hadith has many corroborating narrations, and with them it is hasan.