It is permissible to sit in a naafil prayer with no excuse according to scholarly consensus, but in that case the reward of one who sits will be half that of one who stands, because of the report narrated by Muslim (1214) from ‘Abd-Allah ibn ‘Amr who said: I was told that the Messenger of Allah (blessings and peace of Allah be upon him) said: “A man’s prayer sitting is half a prayer.” 

He said: I came to him and found him praying sitting down. I put my hand on his head and he said: “What is the matter with you, O ‘Abd-Allah ibn ‘Amr?” 

I said: O Messenger of Allah, I was told that you said: “A man’s prayer sitting is half a prayer,” but you are praying sitting down? He said: “Yes, but I am not like one of you.” 

If he prays sitting down because of an excuse, he will have the reward of one who prays standing. 

Al-Nawawi (may Allah have mercy on him) said in Sharh Muslim: What is meant is: the prayer of one who sits down whilst praying is half the reward of the one who prays standing. This implies that it is valid but its reward is reduced. This hadeeth is to be understood as referring to offering naafil prayers when one is able to stand; he will have half the reward of one who prays standing. But if a person offers a naafil prayer sitting down because he is unable to stand, his reward will not be reduced; rather he will have the same reward as one who prays standing. But with regard to the obligatory prayer, if a person prays sitting when he is able to stand, it is not valid and he will not have any reward for it; rather he is sinning by doing that. 

Our companions said: if he regards doing that as permissible, this is kufr and the rulings on apostates apply to him, as is the case if he regards zina, riba and other known haraam things as permissible. 

If he offers an obligatory prayer sitting because he is unable to stand, or he does so lying down because he is unable to stand or sit, then his reward will be like that of one who prays standing and it will not be reduced, according to the consensus of our companions. Thus the hadeeth is to be understood as discussing the issue of reward for one who offers a naafil prayer sitting when he is able to stand. This is our point of view in detail and it is the view of the majority with regard to the interpretation of this hadeeth. 

And he said: With regard to the words of the Prophet (blessings and peace of Allah be upon him), “I am not like one of you,” according to our companions this is one of the unique characteristics of the Prophet (blessings and peace of Allah be upon him); his naafil prayer offered sitting when he was able to stand was like his naafil prayer offered standing as an honour to him, as he was also singled out for things that are well known in the books of our companions and others, which I have listed at the beginning of my book Tahdheeb al-Asma’ wa’l-Lughaat.  

End quote from Sharh Saheeh Muslim, 6/14 

Based on that: 

You can say the opening takbeer standing and then sit, or you may say the opening takbeer whilst sitting, then when you feel more energetic you can get up, and this is better and preferable. The Prophet (blessings and peace of Allah be upon him) used to pray at night, and when he got tired he would sit down, then when he was going to bow, he would get up and bow. 

It was narrated that ‘Aa’ishah (may Allah be pleased with her) said: I did not see the Messenger of Allaah (blessings and peace of Allah be upon him) reciting in any night prayer whilst sitting until he grew old, then he would recite whilst sitting, and when there were thirty or forty verses left of the soorah, he would stand up and recite them, then he would bow.

Narrated by al-Bukhaari (1148) and Muslim (731). 

Al-Nawawi (may Allah have mercy on him) said in Sharh Muslim: This indicates that it is permissible in the same rak‘ah to stand for part of it and sit for part of it. This is our opinion and the opinion of Maalik, Abu Haneefah and most of the scholars. It is the same whether one stands then sits, or sits then stands. Some of the early scholars disallowed that, but that is extreme. Al-Qaadi narrated from Abu Yoosuf and Muhammad, the two companions of Abu Haneefah, among others, that it is makrooh to sit after standing, but if the person intended to stand, then he wanted to sit down, that is permissible in our view and that of the majority. Among the Maalikis, Ibn al-Qaasim regarded it as permissible and Ashhab disallowed it. End quote. 

And Allah knows best.