مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 312, 313
Purification
Circumstances which make ablution necessary - Section 2
وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم: «مِفْتَاحُ الصَّلَاةِ الطُّهُورُ وَتَحْرِيمُهَا التَّكْبِيرُ وَتَحْلِيلُهَا التَّسْلِيمُ» . رَوَاهُ أَبُو دَاوُد وَالتِّرْمِذِيّ والدارمي وَرَوَاهُ ابْنُ مَاجَهْ عَنْهُ وَعَنْ أَبِي سَعِيدٍ
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, and Ibn Majah transmitted it both from 'Ali and from Abu Sa'id.
Mishkat al-Masabih, 312, 313حسن, حسن (الألباني)