Al-Mundhiri records in his book al-Targhib wal-Tarhib:

الترغيب في الاستغفار

Encouragement to seek forgiveness from Allah.

Source: al-Targhīb wal-Tarhīb 2/308

Anas ibn Malik reported: The Messenger of Allah, peace and blessings be upon him, said:

قَالَ اللَّهُ تَبَارَكَ وَتَعَالَى يَا ابْنَ آدَمَ إِنَّكَ مَا دَعَوْتَنِي وَرَجَوْتَنِي غَفَرْتُ لَكَ عَلَى مَا كَانَ فِيكَ وَلَا أُبَالِي يَا ابْنَ آدَمَ لَوْ بَلَغَتْ ذُنُوبُكَ عَنَانَ السَّمَاءِ ثُمَّ اسْتَغْفَرْتَنِي غَفَرْتُ لَكَ وَلَا أُبَالِي يَا ابْنَ آدَمَ إِنَّكَ لَوْ أَتَيْتَنِي بِقُرَابِ الْأَرْضِ خَطَايَا ثُمَّ لَقِيتَنِي لَا تُشْرِكُ بِي شَيْئًا لَأَتَيْتُكَ بِقُرَابِهَا مَغْفِرَةً

Allah Almighty said: O son of Adam, if you call upon Me and place your hope in Me, I will forgive you without hesitation. O son of Adam, if you have sins piling up to the clouds and then ask for My forgiveness, I will forgive you without hesitation. O son of Adam, if you come to Me with enough sins to fill the earth and then you meet Me without associating anything with Me, I will come to you with enough forgiveness to fill the earth.

Source: Sunan al-Tirmidhī 3540, Grade: Sahih

Abu Sa’id al-Khudri reported: The Messenger of Allah, peace and blessings be upon him, said,

إِنَّ الشَّيْطَانَ قَالَ وَعِزَّتِكَ يَا رَبِّ لَا أَبْرَحُ أُغْوِي عِبَادَكَ مَا دَامَتْ أَرْوَاحُهُمْ فِي أَجْسَادِهِمْ قَالَ الرَّبُّ وَعِزَّتِي وَجَلَالِي لَا أَزَالُ أَغْفِرُ لَهُمْ مَا اسْتَغْفَرُونِي

Satan said: By Your might, O Lord, I will continue to mislead the children of Adam, as long as their souls are in their bodies. The Lord said: By My might and majesty, I will continue to forgive them, as long as they seek My forgiveness.”

Source: Musnad Aḥmad 27627, Grade: Sahih

Abdullah ibn Busr reported: The Prophet, peace and blessings be upon him, said:

طُوبَى لِمَنْ وَجَدَ فِي صَحِيفَتِهِ اسْتِغْفَارًا كَثِيرًا

Blessed is he who finds many prayers for forgiveness in his record.

Source: Sunan Ibn Mājah 3818, Grade: Sahih

Al-Zubayr reported: The Messenger of Allah, peace and blessings be upon him, said:

مَنْ أَحَبَّ أَنْ تَسُرُّهُ صَحِيفَتُهُ فَلْيُكْثِرْ فِيهَا مِنَ الِاسْتِغْفَارِ

Whoever would love to be pleased with his record, let him increase seeking forgiveness on it.

Source: Shu’ab al-Imān 639, Grade: Sahih

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

إِنَّ الْعَبْدَ إِذَا أَخْطَأَ خَطِيئَةً نُكِتَتْ فِي قَلْبِهِ نُكْتَةٌ سَوْدَاءُ فَإِذَا هُوَ نَزَعَ وَاسْتَغْفَرَ وَتَابَ سُقِلَ قَلْبُهُ وَإِنْ عَادَ زِيدَ فِيهَا حَتَّى تَعْلُوَ قَلْبَهُ وَهُوَ الرَّانُ الَّذِي ذَكَرَ اللَّهُ كَلَّا بَلْ رَانَ عَلَى قُلُوبِهِمْ مَا كَانُوا يَكْسِبُونَ

Verily, when the servant commits a sin, a black mark appears upon his heart. If he abandons the sin, seeks forgiveness, and repents, then his heart will be polished. If he returns to the sin, the blackness will be increased until it overcomes his heart. It is the covering that Allah has mentioned: No, rather a covering is over their hearts from what they have earned. (83:14)

Source: Sunan al-Tirmidhī 3334, Grade: Sahih

Abu Bakr reported: The Prophet, peace and blessings be upon him, said:

مَا مِنْ رَجُلٍ يُذْنِبُ ذَنْبًا ثُمَّ يَقُومُ فَيَتَطَهَّرُ ثُمَّ يُصَلِّي ثُمَّ يَسْتَغْفِرُ اللَّهَ إِلَّا غَفَرَ اللَّهُ لَهُ

No man commits a sin, then purifies himself in ablution, then prays, and then asks Allah for forgiveness, but that Allah will forgive him.

Then, the Prophet recited this verse:

وَالَّذِينَ إِذَا فَعَلُوا فَاحِشَةً أَوْ ظَلَمُوا أَنفُسَهُمْ ذَكَرُوا اللَّهَ فَاسْتَغْفَرُوا لِذُنُوبِهِمْ وَمَن يَغْفِرُ الذُّنُوبَ إِلَّا اللَّهُ وَلَمْ يُصِرُّوا عَلَىٰ مَا فَعَلُوا وَهُمْ يَعْلَمُونَ

Those who, if they behave immorally or wrong themselves, they remember Allah and seek forgiveness for their sins. Who forgives sins besides Allah? And they do not insist on what they have done, while they know better. (3:135)

Source: Sunan al-Tirmidhī 406, Grade: Sahih

Zayd ibn Harithah reported: The Messenger of Allah, peace and blessings be upon him, said,

مَنْ قَالَ أَسْتَغْفِرُ اللَّهَ الْعَظِيمَ الَّذِي لَا إِلَهَ إِلَّا هُوَ الْحَيَّ الْقَيُّومَ وَأَتُوبُ إِلَيْهِ غُفِرَ لَهُ وَإِنْ كَانَ فَرَّ مِنْ الزَّحْفِ

Whoever says this supplication, ‘I seek forgiveness from Allah Almighty, besides whom there is no God, the Living, the Sustainer, and I repent to him,’ then Allah will forgive him even if he fled from battle.

Source: Sunan al-Tirmidhī 3577, Grade: Sahih

Al-Bara’ reported: A man said to him, “O Abu ‘Umarah, the verse: Do not throw yourself to destructions by your own hands (2:195). Is it a man who meets the enemy in battle and he fights until he is killed?” Al-Bara’ said:

لَا وَلَكِنْ هُوَ الرَّجُلُ يُذْنِبُ الذَّنْبَ فَيَقُولُ لَا يَغْفِرُهُ اللهُ لِي

No, rather it is that a man commits a sin and he says: Allah will not forgive me.

Source: Shu’ab al-Imān 6691, Grade: Sahih li ghayrihi