The verse, and the verse straight after it
The first of these is usually posted on its own, and the one that follows it is usually left off. It should be both or neither. Read them as a pair, and notice which one comes first.
Qur’an and Sunnah
يَٰبَنِيَّ ٱذۡهَبُواْ فَتَحَسَّسُواْ مِن يُوسُفَ وَأَخِيهِ وَلَا تَاْيۡـَٔسُواْ مِن رَّوۡحِ ٱللَّهِۖ إِنَّهُۥ لَا يَاْيۡـَٔسُ مِن رَّوۡحِ ٱللَّهِ إِلَّا ٱلۡقَوۡمُ ٱلۡكَٰفِرُونَ
My sons, go and seek news of Joseph and his brother and do not despair of God’s mercy- only disbelievers despair of God’s mercy.’
Qur’an 12:87
قَالَ وَمَن يَقۡنَطُ مِن رَّحۡمَةِ رَبِّهِۦٓ إِلَّا ٱلضَّآلُّونَ
He said, ‘Who but the misguided despair of the mercy of their Lord?’
Qur’an 15:56
وَٱلَّذِينَ إِذَا فَعَلُواْ فَٰحِشَةً أَوۡ ظَلَمُوٓاْ أَنفُسَهُمۡ ذَكَرُواْ ٱللَّهَ فَٱسۡتَغۡفَرُواْ لِذُنُوبِهِمۡ وَمَن يَغۡفِرُ ٱلذُّنُوبَ إِلَّا ٱللَّهُ وَلَمۡ يُصِرُّواْ عَلَىٰ مَا فَعَلُواْ وَهُمۡ يَعۡلَمُونَ
those who remember God and implore forgiveness for their sins if they do something shameful or wrong themselves- who forgives sins but God?- and who never knowingly persist in doing wrong
Qur’an 3:135
۞قُلۡ يَٰعِبَادِيَ ٱلَّذِينَ أَسۡرَفُواْ عَلَىٰٓ أَنفُسِهِمۡ لَا تَقۡنَطُواْ مِن رَّحۡمَةِ ٱللَّهِۚ إِنَّ ٱللَّهَ يَغۡفِرُ ٱلذُّنُوبَ جَمِيعًاۚ إِنَّهُۥ هُوَ ٱلۡغَفُورُ ٱلرَّحِيمُ
Say, ‘[God says], My servants who have harmed yourselves by your own excess, do not despair of God’s mercy. God forgives all sins: He is truly the Most Forgiving, the Most Merciful
Qur’an 39:53
وَأَنِيبُوٓاْ إِلَىٰ رَبِّكُمۡ وَأَسۡلِمُواْ لَهُۥ مِن قَبۡلِ أَن يَأۡتِيَكُمُ ٱلۡعَذَابُ ثُمَّ لَا تُنصَرُونَ
Turn to your Lord. Submit to Him before the punishment overtakes you and you can no longer be helped
Qur’an 39:54
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.