Bismillaah:

Mihjan ibn al-Adra رضي الله عنه relates:

The Messenger of Allaah صلي الله عليه وسلم entered the masjid and saw a man who had finished his prayer, and was reciting the tashahhud saying:

اللَّهُمَّ إِنِّي أَسْأَلُكَ يَا اللَّهُ الأَحَدُ الصَّمَدُ الَّذِي لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ أَنْ تَغْفِرَ لِي ذُنُوبِي إِنَّكَ أَنْتَ الْغَفُورُ الرَّحِيمُ ‏.

"Allaahumma, inni as'aluka ya Allaah! Al Ahad us-Samad, Alazi lam yalid wa lam yulad, wa lamyakun lahu kufuwan ahad, an taghfirali Thunubi, innaka antal Ghafur ur Raheem."

" O Allaah, I ask of You, O Allaah, as You are the One, the Only, the self-sufficient Master, Who begets not nor was he begotten, and there is none equal, or comparable to Him, forgive me my sins, for You are the Oft-Forgiving, Most Merciful."

The Messenger of Allaah صلي الله عليه وسلم) said: He has been forgiven (repeating three times.)

Takhreej:

{ Imam Abu Dawud and An Nasaa'i in their Sunnans and graded as Saheeh by Al Hakim, Ad Dahaabi, Ibn Khuzaimah, Al Albaani, Abu Taahir and others }

Shaykh Abu Mu'aadh Al Afghaani رحمه الله commented:

This has been an immensely great good tiding not only for Mihjan رضي الله عنه, but everyone who supplicates in this manner. This Supplication consists of the supremely great name of Allaah عزوجل (Al-lsmu'l A'zam), because the attributes mentioned therein belong to the unique self of Allaah, Most High. None other has even a slightest tinge of it.

{ Al Haashiyah ala Sunnan An Nasaa'i }

*** And Allaah عزوجل knows best ***