What is the ruling on supplicating with the words: "O Allah, unleash upon them a dog from among Your dogs" in the Qunut, and what is the ruling on the prayer of a congregant behind someone who supplicates with this supplication?

There is no harm in supplicating with "O Allah, unleash Your dog upon him" against the enemies of Allah and His religion, as the Prophet, peace and blessings be upon him, supplicated with this against his paternal uncle, Abu Lahab. Al-Hakim and Al-Bayhaqi narrated that Lahab ibn Abi Lahab used to insult the Prophet, peace and blessings be upon him, so the Prophet, peace and blessings be upon him, said: (O Allah, unleash Your dog upon him). Then a lion came and tore him apart. Al-Hafiz Ibn Hajar and others graded the hadith as hasan (good). It should be known that it may be appropriate to supplicate for the destruction of a disbeliever or for their guidance, depending on the severity of their enmity towards Muslims. Sheikh Ibn Uthaymeen, may Allah have mercy on him, was asked about supplicating against apostates and disbelievers, and he replied: If the apostate is harmful, then there is no harm in supplicating for their destruction to ward off their evil. If they are not harmful, then it is better to supplicate for their guidance. The duty of the rulers towards apostates is to invite them to Islam and give them respite. If they persist, then they must be killed, according to the saying of the Prophet, peace and blessings be upon him: (Whoever changes his religion, kill him).