Abu Dawud Book 2, Hadith Number 964.
Narated By Abdullah ibn Mas’ud : We did not know what we should say when we sat during prayer. The Apostle of Allah (PBUH) 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.