It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) forbade leaning on one's left hand while sitting in the prayer, and said: "This is the prayer of the Jews."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2810