It was narrated from Muhammed bin Al-Hanafiyyah that his father said:
"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.'" (ES)
English reference (ES): Vol. 1, Book 1, Hadith 275 (ES)
Arabic reference (ES): Book 1, Hadith 288 (ES)