(a) It is transmitted from Sayyiduna Ali (may Allah be pleased with him), in the explanation of "So pray to your Lord and sacrifice" [Surah al-Kawthar: 2], that what is meant is placing one's right hand on the middle of the forearm, on the chest.
(b) Ali (may Allah be pleased with him) said: What is meant is placing them on the edge of the wrist.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2332