Sunan an-Nasa'i, Hadith 1039 ⓘ
It was narrated that Abu Humaid As-Sa'idi said: "When the Prophet bowed he was balanced, he did not make his head higher or lower than his back, and he put his hands on his knees."
أَخْبَرَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا يَحْيَى، قَالَ حَدَّثَنَا عَبْدُ الْحَمِيدِ بْنُ جَعْفَرٍ، قَالَ حَدَّثَنِي مُحَمَّدُ بْنُ عَمْرِو بْنِ عَطَاءٍ، عَنْ أَبِي حُمَيْدٍ السَّاعِدِيِّ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم إِذَا رَكَعَ اعْتَدَلَ فَلَمْ يَنْصِبْ رَأْسَهُ وَلَمْ يُقْنِعْهُ وَوَضَعَ يَدَيْهِ عَلَى رُكْبَتَيْهِ .
also referenced under: Book 12, Hadith 11 (In-Book) & Volume 2, Book 12, Hadith 1040 (DarusSalam)