There is no doubt that the Sunnah is for the worshipper to place his right hand over his left when praying.
It was narrated that Sahl ibn Sa’d (may Allaah be pleased with him) said: The people were commanded to place the right hand over the left forearm when praying.
Abu Haazim said: All I know is that he attributed this to the Prophet (peace and blessings of Allaah be upon him). Narrated by al-Bukhaari, 707.
It was narrated from Waa’il ibn Hujr (may Allaah be pleased with him) that he saw the Prophet (peace and blessings of Allaah be upon him) raising his hands level with his ears when he started to pray and saying takbeer, then he wrapped himself in his garment and placed his right hand on his left. Narrated by Muslim, 401.
See also the answer to question no. 5770 for information on the true nature of the religion of the Raafidis, in which we have quoted the saheeh evidence for placing the right hand on the left in prayer.
In the answer to question no. 6109 you will find a refutation of those who quote Imam Maalik as going against this Sunnah, in which some senior Maalikis clearly affirm that this is the Sunnah of the Prophet (peace and blessings of Allaah be upon him) and was the way of Imam Maalik.
If a person feels tired, he may let his arms drop for as long as he needs to rest them, then he should go back to holding the right hand over the left. Allaah says (interpretation of the meaning):
“Allaah burdens not a person beyond his scope”
The Prophet (peace and blessings of Allaah be upon him) used to pray at night and when he got tired he would sit, then when he was about to bow he would stand up so that he could bow.
It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: When I saw the Prophet (peace and blessings of Allaah be upon him) reciting during night prayers sitting down, he would say takbeer and recite sitting down, then when there were thirty or forty verses left, he would stand up and recite them, then he would bow. Narrated by al-Bukhaari, 1097; Muslim, 731.
It is narrated in al-Saheehayn that the Prophet (peace and blessings of Allaah be upon him) prayed sitting for a few days because he was sick. If it is permissible not to stand, which is one of the pillars or essential parts of the prayer, because of sickness, then it is more appropriate that it should be permissible not to do one of the Sunnahs of prayer because of tiredness, on condition that one go back to holding the right hand over the left when that tiredness is no longer present.
Al-Shaafa’i (may Allaah have mercy on him) said in al-Umm (1/100):
If a person starts the prayer standing then some excuse appears, he may sit, but if that excuse then disappears, he is not allowed to do anything but stand. End quote.
Al-Nawawi said in Sharh Muslim, 6/11:
‘Aa’ishah’s words, “[he would] recite sitting down, then when there were thirty or forty verses left, he would stand up and recite them, then he would bow” indicate that it is permissible to sit during part of a rak’ah and to stand during part of the same rak’ah. This is our view and is the view of Maalik, Abu Haneefah and most of the scholars, whether one stands then sits, or sits then stands. Some of the salaf disallowed it, but this is wrong. End quote.
To sum up our answer:
There is nothing wrong with letting the arms hang by the sides during prayer because of tiredness, so long as one goes back to holding the right hand over the left once the tiredness has gone. That is not regarded as imitating the Raafidis, because it is done temporarily and for a reason. It is only imitating them if the worshipper takes it as a habit and lets his arms hang by his sides in every prayer, and never holds the right hand over the left.
And Allaah knows best.