The same narration is reported through another chain; in it, Muhammad bin Amr bin Ata' reports: I heard Sayyiduna Abu Humayd al-Sa'idi (may Allah be pleased with him) say: "When the Messenger of Allah (peace and blessings be upon him) stood for prayer." Then he mentioned the complete hadith, and in it there is: "Then he went down to the ground in prostration, kept his arms away from his sides, and kept space between the toes of his feet."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2718