It was narrated from Abu Sa'eed Al-Khudri that:
The Messenger of Allah said: "The key to prayer is purification, its opening is to say Allahu Akbar and its closing is to say As-salamu 'alaikum." (FR)
English reference (FR): Vol. 1, Book 1, Hadith 276 (FR)
Arabic reference (FR): Book 1, Hadith 289 (FR)