It is narrated from Sayyiduna Ali (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: The key to prayer is ablution, its sanctification is Allahu Akbar, and its conclusion is Salam.
Hadith Referenceالفتح الربانی / أبواب صفة الصلاة / 1837
Sayyida Aisha (may Allah be pleased with her) describing the night prayer of the Messenger of Allah (peace and blessings be upon him) says: Then he (peace be upon him) would sit and recite the Tashahhud and supplicate, and then say "As-salamu ‘alaykum" turning his face to one side, and would raise his voice enough to wake us up.
Hadith Referenceالفتح الربانی / أبواب صفة الصلاة / 1838