It is narrated from Sayyiduna Wa'il bin Hujr (may Allah be pleased with him); he said: "I saw the Prophet (peace and blessings be upon him) enter into the prayer; he (peace and blessings be upon him) said the takbir, then placed his right forearm upon his left forearm."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18270