(a) It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the people used to be commanded "to place the right hand on the left forearm in the prayer (while standing)."
(b) Abu Hazim (may Allah have mercy on him) said: I know only that he attributed this (to the Prophet), or he said a word like it.
(c) Imam al-Bukhari (may Allah have mercy on him) narrated it in his Sahih from al-Qa'nabi, and said that Abu Hazim (may Allah have mercy on him) said: I know nothing except that he traced it back to the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2326