All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad sallallaahu alayhi wa sallam is His slave and Messenger.
Putting right hand over the left hand during prayer is Sunnah of the Prophet Muhammad sallallaahu alayhi wa sallam this Sunnah is proven through more than twenty narrations. But where the hands should be placed is a controversial issue among Muslim scholars. According to the Hanafi school of jurisprudence one's hands should be kept below navel while Shaafi'i school of jurisprudence confirms that they should be placed on chest, and both practices are narrated from Imaam Ahmad . Imaam At-Tirmithi said: 'The people of knowledge from the companions their successors and the scholars who came after them are agreed that right hand should be placed over left hand during prayers. Some of them are of the opinion that the hands should be placed above the navel while others are of the opinion that they should be placed below the navel. Both practices are acceptable'.
Halab Al-Taa'i narrated: 'I saw the Prophet sallallaahu alayhi wa sallam praying with his right hand over his left upon his chest'. This is related by Ahmad and At-Tirmithi who graded it as Hasan. In another narration reported Waa'il Ibn Hujr : "Once when I prayed with the Prophet sallallaahu alayhi wa sallam, he sallallaahu alayhi wa sallam placed his right hand over his left placing them over his chest." The report is recorded by Ibn Khuzaymah who considers it as Saheeh, and by Abu Daawood and An-Nasaa'i with the wording, "Then he put his right hand over the back of his left wrist and forearm."
Know that four the Imaams have knowledge of Islamic jurisprudence, and hold the high rank of piety, and they never go against an authentic Sunnah after having knowledge of it and being certain of its authenticity.
However, if a narration comes to their knowledge and they rule it as Saheeh but they do not practice it, it may be for many reasons. Sometime they assume that some other narrations contradict it or they may be suspicious of its authenticity, etc. In this case, they will try their best to adopt an opinion, which accommodates other evidences. They do not intend to ignore an authentic Sunnah.
If we know an authentic Sunnah and at the same time we know that an eminent Imaam or scholar does not implement it, it means that the Sunnah did not come to his knowledge.
Also, there is nothing strange for a known Imaam not to have knowledge of a certain narration, since the companions of the Prophet Muhammad sallallaahu alayhi wa sallam were spread in various places and lived in different areas.
Sometimes many had the knowledge of something while the others did not have it. Thus, for one person to have all the knowledge of the companions of the Prophet is difficult, if it is not impossible. The evidence for this is that Abu Bakr As-Siddeeq who used to be with the Prophet sallallaahu alayhi wa sallam more than anyone else and who was the nearest person to the Prophet sallallaahu alayhi wa sallam, did not have the knowledge about the share of a grandmother in the inheritance. When a grandmother asked him for her share of inheritance, he said: "There is nothing prescribed for you in the Book of Allaah, nor do I know anything for you according to the Sunnah of the Prophet of Allaah sallallaahu alayhi wa sallam. Go home till I question the people (of knowledge). He then questioned other companions, and Al-Mugheerah Ibn Shu'bah said: "I was present with the Apostle of Allaah sallallaahu alayhi wa sallam when he gave the grandmother the sixth". Abu Bakr said: "Was there anyone with you?" Muhammad Ibn Maslamah stood and said the same as Al-Mugheerah Ibn Shu'bah had said. Thereupon, Abu Bakr made applied this rule to her.
It is narrated by Imaams Al-Bukhari and Muslim that Faatimah sent somebody to Abu Bakr asking him to give her share of the inheritance from her father, i.e. the Prophet sallallaahu alayhi wa sallam, that means she was not aware that the Prophet's property is not to be inherited till Abu Bakr made it clear to her and said: "Allaah's Apostle sallallaahu alayhi wa sallam said: 'We (Prophets), our property is not inherited, and whatever we leave is charity.
If Abu Bakr with all his merits and sacrifices, was not aware of a Sunnah and Fatimah the daughter of the Prophet sallallaahu alayhi wa sallam and wife of Ali Ibn Abu Taalib was not aware of a narration, then it is very possible that a companion or an Imaam might not have had knowledge of a given narration.
Allaah knows best.