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 is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It is Sunnah for the people praying behind the Imaam to say 'Aameen' when he supplicates during prayer (Witr). 'Aameen' means: 'O, Allaah, answer our supplication.'
However, it is not confirmed in the Sunnah that the people praying behind the Imaam used to say "O, Allaah", or "We testify" or "Truly", when he praises Allaah while supplicating. For instance, when the Imaam says "O, Allaah those people whom you honor will not be humiliated, and those whom you are against will not be glorified." People imitate each other in saying the above words and other terms without having a proof from the Sunnah.
Some of the jurists are of the view that whoever says these words, his prayer is invalid because they are not considered to be remembrance of Allaah that should be said during the prayer, as they were not confirmed in the Sunnah.
Shaykh Ibn Uthaymeen considered these words "We testify", "O, Allaah", "Truly" as innovations.
Therefore, the people praying behind the Imaam should remain silent when the Imaam is praising Allaah. However, some jurists are of the view that the people who are led in prayer should say the same thing as the Imaam when he praises Allaah.
An-Nawawi said: 'It is more appropriate to imitate the Imaam because the words that he says are praises and remembrances of Allaah, and saying "Aameen" is not appropriate.'
As regards raising hands when supplicating in prayer, the scholars differed about it. The most preponderant opinion, however, is that it is desirable to raise the hands. This is the opinion of Hanbali, Shaafi'ee, and Hanafi schools. The evidence for this is the narration of Anas that "the Prophet raised his hands while making supplications against those who killed the reciters of the Qur'aan in Maouna well. It is also confirmed that 'Umar raised his hands in Witr prayer whilst supplicating." [Al-Bayhaqi]
Allaah knows the best.