6:147-ref

Allah often joins encouragement with threats in the Qur'an. Allah said at the end of this Surah:

إِنَّ رَبَّكَ سَرِيعُ الْعِقَابِ وَإِنَّهُ لَغَفُورٌ رَّحِيمٌ

Surely, your Lord is swift in retribution, and certainly He is Oft-Forgiving, Most Merciful. (6:165)

Allah also said,

وَإِنَّ رَبَّكَ لَذُو مَغْفِرَةٍ لِّلنَّاسِ عَلَى ظُلْمِهِمْ وَإِنَّ رَبَّكَ لَشَدِيدُ الْعِقَابِ

But verily, your Lord is full of forgiveness for mankind in spite of their wrongdoing. And verily, your Lord is (also) severe in punishment. (13:6)

and,

نَبِّىءْ عِبَادِى أَنِّى أَنَا الْغَفُورُ الرَّحِيمُ وَأَنَّ عَذَابِى هُوَ الْعَذَابُ الاٌّلِيمُ

Declare unto My servants, that truly, I am the Oft-Forgiving, the Most Merciful. And that My torment is indeed the most painful torment. (15:49-50)

and,

غَافِرِ الذَّنبِ وَقَابِلِ التَّوْبِ شَدِيدِ الْعِقَابِ

The Forgiver of sin, the Acceptor of repentance, the Severe in punishment. (40:3)

and,

إِنَّ بَطْشَ رَبِّكَ لَشَدِيدٌ إِنَّهُ هُوَ يُبْدِىءُ وَيُعِيدُ وَهُوَ الْغَفُورُ الْوَدُودُ

Verily, the punishment of your Lord is severe and painful. Verily, He it is Who begins and repeats. And He is Oft-Forgiving, full of love. (85:12-14)

There are many other Ayat on this subject.