What is the ruling on seeking forgiveness by saying: "I seek forgiveness from Allah, the Almighty, Whom there is none worthy of worship except Him, the Living, the Sustainer, and to Him I repent, from the sins I know and from the sins I do not know," with the intention of repenting from all sins?

Seeking forgiveness (Istighfar) is among the best acts of obedience and the most excellent means of drawing closer to Allah. You have chosen well by selecting, "Astaghfirullah alladhi la ilaha illa Huwal-Hayyul-Qayyumu wa atubu ilaih" (I seek forgiveness from Allah, other than Whom there is no deity, the Ever-Living, the Sustainer, and I repent to Him), for the Prophet, peace and blessings be upon him, said: "Whoever says: 'Astaghfirullah alladhi la ilaha illa Huwal-Hayyul-Qayyumu wa atubu ilaih,' his sins will be forgiven, even if he had fled from the battlefield."