There are two ways of placing the right hand on the left hand when praying: 

1 – The right hand is placed on the left hand, wrist and forearm. 

Abu Dawood (726) and al-Nasaa’i (889) narrated that Waa’il ibn Hujr said: I said: I am going to watch how the Messenger of Allaah (peace and blessings of Allaah be upon him) prays. So I watched him, and he stood and said takbeer, raising his hands until they were level with his ears. Then he placed his right hand on his left hand, wrist and forearm… 

Classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

Al-Sindi said in Haashiyat al-Nasaa’i: 

Then he placed his right hand on the back of his left hand, wrist and forearm. 

The wrist is the joint between the hand and forearm. What is meant is that he placed his hand so that the middle of his right hand was on the wrist, which means that part of it was on the left hand and part was on the forearm. End quote. 

2 – The right hand is clasped over the left. 

Al-Nasaa’i (88) narrated that Waa’il ibn Hujr (may Allaah be pleased with him) said: I saw the Messenger of Allaah (peace and blessings of Allaah be upon him) when he was standing in prayer; he clasped his right hand over his left. 

Classed as saheeh by al-Albaani in Saheeh al-Nasaa’i. 

Al-Albaani (may Allaah have mercy on him) said in Sifat Salaat al-Nabi (peace and blessings of Allaah be upon him) (The Prophet’s Prayer Described) p. 68: 

He would place his right hand over the back of his left hand, wrist and forearm, and he told his companions to do likewise. Sometimes he would clasp his right hand over his left, and place them on his chest. End quote.