Raising the hands in the Qunoot an-Naazilah is established from the Prophet (ﷺ). It comes on the authority of Anas ibn Malik (may Allah be pleased with him) that he said:
، فلقدْ رأيتُ رسولَ اللهِ صلَّى اللهُ عليه وسلم في صلاةِ الغَداةِ رفع يديهِ فدعا عليهم . وفي روايةٍ : يدعو عليهم
I saw the Messenger of Allah (ﷺ) in the Fajr prayer raise his hands and supplicate against them. In one version: supplicating against them [i.e. those disbelievers who killed seventy reciters from the Companions]
[Graded Saheeh by Imam al-Albani in Asl Sifat-us-Salah (3/957)]
It is also established from Umar ibn al-Khattab and Ibn Abbas (may Allah be pleased with them) that they raised their hands in the Qunoot.
As for those behind the Imam, then they too raise their hands.
Shaikh al-Albani (may Allah have mercy upon him) said: “Raising the hands in the Qunoot, whether Qunoot of witr or the Qunoot of Fajr or other than it from the five prayers during the time of calamity (Naazilah), then it is allowed to raise the hands in it. If the Imam raises his hands and raises his voice in supplication, then those behind him say Ameen and also raise their hands…” [Silsilatul-Huda wan-Nur no. 703]
And Allah Knows Best
Faisal Ibn Abdul Qaadir Ibn Hassan