The benefit of uttering Sayyidul Istigfar

وَعَنْ شَدَّادِ بْنِ أَوْسٍ ‏- رَضِىَ اللهُ عَنْهُ ‏- قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ ‏- سَيِّدُ الْاِسْتِغْفَارِ أَنْ تَقُولَ : اَللّٰهُمَّ أَنْتَ رَبِّيْ لَا إِلٰهَ إِلَّا أَنْتَ خَلَقْتَنِيْ وَأَنَا عَبْدُكَ وَأَنَا عَلٰى عَهْدِكَ وَوَعْدِكَ مَا اسْتَطَعْتُ أَعُوذُ بِكَ مِنْ شَرِّ مَا صَنَعْتُ أَبُوْءُ لَكَ بِنِعْمَتِكَ عَلَيَّ وَأَبُوْءُ لَكَ بِذَنْبِيْ فَاغْفِرْ لِيْ فَإِنَّهُ لَا يَغْفِرُ اَلذُُّنُوبَ إِلَّا أَنْتَ قالَ: ومَنْ قالَهَا مِنَ النَّهارِ مُوقِنًا بِهَا، فَمَاتَ مِنْ يَومِهِ قَبْلَ أنْ يُمْسِيَ فَهُوَ مِنْ أَهْلِ الجَنَّةِ، ومَنْ قَالَهَا مِنَ اللَّيْلِ وَهُوَ مُوْقِنٌ بِهَا، فَمَاتَ قَبْلَ أنْ يُصْبِحَ، فَهُوَ مِنْ أهْلِ الجَنَّةِ
الراوي : شداد بن أوس | المحدث : البخاري | المصدر : صحيح البخاري الصفحة أو الرقم: 6306 | خلاصة حكم المحدث : [ صحيح ]

Hazrat Shaddad ibn Aws ( رضى الله عنه ) narrated: The messenger of Allah ( صلى الله عليه وسلم) said, the best way to seek forgiveness is to say: “O Allah, You are my Lord, there is no god but You. You created me and I am Your servant, and I abide by Your covenant and promise as much as I am able. I seek refuge in You from the evil of what I have done. I acknowledge Your blessings upon me, and I acknowledge my sin, so forgive me, for no one forgives sins except You.” He said: “Whoever says it during the day, believing in its truth, and dies on that day before evening, he is among the people of Paradise. And whoever says it at night, believing in its truth, and dies before morning, he is among the people of Paradise.”
Narrated by: Shaddad ibn Aws | Narrator: Al-Bukhari | Source: Sahih Al-Bukhari Page or number: 6306 | Summary of the narrator’s ruling: [ Sahih ]

Allahu tawala spreads his hands to accept the tawbah until the sun rises from the west.

وَعَنْ أَبِيْ مُوْسَى، رَضِيَ اللهُ عَنْهُ، عَنِ النَّبِيِّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ ‏ ‏إِنَّ اللهَ تَعَالى يَبْسُطُ يَدَهُ بِاللَّيْلِ لِيَتُوْبَ مُسِئُ النَّهَارِ، وَيَبْسُطُ يَدَهُ بِالنَّهَارِ لِيَتُوْبَ مُسِئُ اللَّيْلِ حَتّٰى تَطْلُعَ الشَّمْسُ مِنْ مَّغْرِبِهَا‏
رواه مسلم
Meaning:
Abu Musa (May Allah be pleased with him) reported: The Prophet (صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ) said, "Allah, the Exalted, stretches His Hand during the night so that those who commit sins by day may repent, and He stretches His Hand in the day so that those who commit sins by night may repent. He keeps doing so until the sun rises from the West".
Muslim
Reference : Riyad as-Salihin 437 In-book reference : Introduction, Hadith 437

Benefit of seeking forgiveness for every muslim