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.'" (ID)
English reference (ID): Vol. 1, Book 1, Hadith 275 (ID)
Arabic reference (ID): Book 1, Hadith 288 (ID)