Mishkat al-Masabih
Mishkat al-Masabih 312, 313
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 that the Messenger of God said: ‘The key to prayer is purification; its consecration begins with saying “God is greatest,” and its release is the salutation of peace.’ Abū Dāwūd, al-Tirmidhī, and al-Dārimī transmitted it. Ibn Mājah transmitted it from him and from Abū Saʿīd.
Translation notes
The accompanying grading lists the report as ḥasan (good). The Arabic’s ‘consecration’ and ‘release’ refer to entering and leaving the formal state of prayer through the opening takbīr and closing salutation.