(a) It is narrated from Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) prayed, he kept his arms spread apart (in prostration).
(b) Abu Zakariyya al-Anbari says: 'Jakhkha al-rajulu fi salatihi' means that a person keeps his arms spread out and holds them away from his body in bowing and prostration.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2709