The Prophet ﷺ said, "The chief of all prayers for forgiveness is that the servant says, 'O Allah, You are my Lord. There is no god but You. You created me, and I am Your servant, and I hold to Your covenant and Your promise as best I can. I acknowledge before You all the blessings You have given me, and I acknowledge before You my sin, so forgive me, for none forgives sins except You.'"He continued, "Whoever says it during the day with firm faith in it and dies that day, will be among the people of Paradise, and whoever says it during the night with firm faith in it and dies that night, will be among the people of Paradise."
حدثنا يحيى بن سعيد عن حسين المعلم قال حدثني عبد الله بن بريدة عن بشير بن كعب
عن النبي ﷺ قال سيد الاستغفار أن يقول العبد اللهم أنت ربي لا إله إلا أنت خلقتني وأنا عبدك وأنا على عهدك ووعدك ما استطعت أبوء لك بالنعمة وأبوء لك بذنبي فاغفر لي إنه لا يغفر الذنوب إلا أنت قال إن قالها بعدما يصبح موقنا بها ثم مات كان من أهل الجنة وإن قالها بعدما يمسي موقنا بها ثم مات كان من أهل الجنة
Its isnad is sahih. Husayn Muallim is Ibn Zakwan, a well-known trustworthy narrator, and Abdullah bin Burayda is Aslami, the trustworthy judge. They praised him, and his hadith is found in the collections of the group. Bashir bin Kaab is Adawi Basri, whom Ibn Hibban deemed trustworthy in 4/73. Ibn Abu Hatim said in Jarh 2/395 that he is well known, and Bukhari indicated his soundness in Kabir 2/132, saying, "When the sweeping plague struck, Bashir bin Kaab Adawi hid himself away and recited the Quran during it, and when he died he was buried in that very place," and he mentioned nothing further about him.The hadith was narrated by Bukhari 11/97, number 6306 (Fath), Tirmidhi 5/467, number 3393, who said it is hasan gharib, and Nasai 8/279, number 5522.