It is transmitted from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that he was praying and had placed his left hand on his right hand, when the Prophet (peace and blessings be upon him) saw him, and he placed his right hand on his left hand.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2327