Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1105 (DarusSalam) ⓘ
It was narrated that Abu Ishaq said: "Al-Bara' described the prostration to us. He places his hands on the ground and raised his posterior and said: 'This is what I saw the Messenger of Allah doing."
أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ الْمَرْوَزِيُّ، قَالَ أَنْبَأَنَا شَرِيكٌ، عَنْ أَبِي إِسْحَاقَ، قَالَ وَصَفَ لَنَا الْبَرَاءُ السُّجُودَ فَوَضَعَ يَدَيْهِ بِالأَرْضِ وَرَفَعَ عَجِيزَتَهُ وَقَالَ هَكَذَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَفْعَلُ .
also referenced under: Hadith 1104 (Reference) & Book 12, Hadith 76 (In-Book)