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." (Using translation from Ibn Mājah 276)
عَنْ عَلِيٍّ ؓ رَفَعَهُ إِلَى النَّبِيِّ ﷺ قَالَ مِفْتَاحُ الصَّلَاةِ الطُّهُورُ وَإِحْرَامُهَا التَّكْبِيرُ وَإِحْلَالُهَا التَّسْلِيمُ