After warning and advising his people for close to a thousand years, Prophet Nuh finally invoked Allah and asked him to forgive him, those who accepted faith with him, and every other Believer who was to come from then on afterwards. After Allah had told him that no one else would accept faith among his people, Prophet Nuh invoked Allah to destroy the Unbelievers for their evil and their stubborn rejection of the truth lest future generations be misguided by them. -Various Tafseer.