It is narrated from Sayyiduna al-Bara' (may Allah be pleased with him) that, while describing the manner of prostration, he spread his hands, kept his back raised, kept his belly away from the ground and his arms away from his sides, and said: "This is how the Messenger of Allah (peace and blessings be upon him) used to prostrate."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2714