بُلُوغُ الْمَرَامِ
Bulugh al-Maram 275
The Book of Prayer
وَعَنْ اِبْنِ عُمَرَ -رَضِيَ اَللَّهُ عَنْهُمَا- { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ يَرْفَعُ يَدَيْهِ حَذْوَ مَنْكِبَيْهِ إِذَا اِفْتَتَحَ اَلصَّلَاةَ , وَإِذَا كَبَّرَ لِلرُّكُوعِ , وَإِذَا رَفَعَ رَأْسَهُ مِنْ اَلرُّكُوعِ } مُتَّفَقٌ عَلَيْهِ [1]
Narrated Ibn 'Umar (RA): The Prophet (ﷺ) used to raise his hands parallel to his shoulders when he started the prayer (by saying Allahu Akbar), when he uttered the Takbir to bow and when he raised his head from the Ruku' (bowing posture). [Agreed upon].
Sunnah.com reference: Book 2, Hadith 155