Sayyiduna Amir bin Sa'd narrates from his father (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) commanded, in the prayer (at the time of prostration), "placing the palms (flat) and keeping the feet upright."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2668