مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 810
Prayer
The Nature of Prayer - Section 3
وَعَنْ أَبِي حُمَيْدٍ السَّاعِدِيِّ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا قَامَ إِلَى الصَّلَاةِ اسْتَقْبَلَ الْقِبْلَةَ وَرَفَعَ يَدَيْهِ وَقَالَ: الله أكبر. رَوَاهُ ابْن مَاجَه
Abu Humaid as-Sa‘idi said that when God’s Messenger stood up to pray, he faced the qibla, raised his hands and said, “God is most great." Ibn Majah transmitted it.
Mishkat al-Masabih, 810صَحِيح (الألباني)