Hadith 312
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم: «مِفْتَاحُ الصَّلَاةِ الطُّهُورُ وَتَحْرِيمُهَا التَّكْبِيرُ وَتَحْلِيلُهَا التَّسْلِيمُ» . رَوَاهُ أَبُو دَاوُد وَالتِّرْمِذِيّ والدارمي
He also reported God's messenger as saying, “The key of prayer is purification, saying ‘God is most great’ makes it a sacred state, and the salutation brings the sacred state to an end.” Abu Dawud, Tirmidhi and Darimi transmitted it,