We did not know what we should say when we sat during prayer. The Messenger of Allah ﷺ was taught (by Allah). He then narrated the tradition to the same effect. Sharik reported from Jami', from AbuWa'il on the authority of Abdullah ibn Mas'ud something similar. He said: He used to teach us also some other words, but he did not teach them as he taught us the tashahhud: O Allah, join our hearts, mend our social relationship, guide us to the path of peace, bring us from darkness to light, save us from obscenities, outward or inward, and bless our ears, our eyes, our hearts, our wives, our children, and relent toward us; Thou art the Relenting, the Merciful. And make us grateful for Thy blessing and make us praise it while accepting it and give it to us in full.  

كُنَّا لاَ نَدْرِي مَا نَقُولُ إِذَا جَلَسْنَا فِي الصَّلاَةِ وَكَانَ رَسُولُ اللَّهِ ﷺ قَدْ عَلِمَ فَذَكَرَ نَحْوَهُ قَالَ شَرِيكٌ وَحَدَّثَنَا جَامِعٌ يَعْنِي ابْنَ شَدَّادٍ عَنْ أَبِي وَائِلٍ عَنْ عَبْدِ اللَّهِ بِمِثْلِهِ قَالَ وَكَانَ يُعَلِّمُنَا كَلِمَاتٍ وَلَمْ يَكُنْ يُعَلِّمُنَاهُنَّ كَمَا يُعَلِّمُنَا التَّشَهُّدَ اللَّهُمَّ أَلِّفْ بَيْنَ قُلُوبِنَا وَأَصْلِحْ ذَاتَ بَيْنِنَا وَاهْدِنَا سُبُلَ السَّلاَمِ وَنَجِّنَا مِنَ الظُّلُمَاتِ إِلَى النُّورِ وَجَنِّبْنَا الْفَوَاحِشَ مَا ظَهَرَ مِنْهَا وَمَا بَطَنَ وَبَارِكْ لَنَا فِي أَسْمَاعِنَا وَأَبْصَارِنَا وَقُلُوبِنَا وَأَزْوَاجِنَا وَذُرِّيَّاتِنَا وَتُبْ عَلَيْنَا إِنَّكَ أَنْتَ التَّوَّابُ الرَّحِيمُ وَاجْعَلْنَا شَاكِرِينَ لِنِعْمَتِكَ مُثْنِينَ بِهَا قَابِلِيهَا وَأَتِمَّهَا عَلَيْنَا