Reference : Hisn al-Muslim 63

اللّهُـمَّ إِنِّـي أَسْأَلُـكَ يا اللهُ 
بِأَنَّـكَ الواحِـدُ الأَحَـد الصَّـمَدُ 
الَّـذي لَـمْ يَلِـدْ وَلَمْ يولَدْ، 
وَلَمْ يَكـنْ لَهُ كُـفُواً أَحَـد ، 
أَنْ تَغْـفِرْ لي ذُنـوبي 
إِنَّـكَ أَنْـتَ الغَفـورُ الرَّحِّـيم
Allāhumma innī as'aluka yā Allāh
bi'annaka ‘l-Wāḥidu ‘l-Aḥaduṣ-ṣamad, 
alladhī lam yalid wa lam yūlad, 
wa lam yakun lahu kufuwan aḥad, 
an taghfir lī dhunūbī, 
innaka anta ‘l-Ghafūrur-Raḥīm.


O Allah, I ask You. O Allah, You are the One, the Only, Self-Sufficient Master, Who was not begotten and begets not, and none is equal to Him. 
Forgive me my sins, surely you are Forgiving, Merciful.

Reference:
An-Nasa'i 3/52, Ahmad 4/338. See also Al-Albani, Sahih An-Nasa'i 1/280 and 
Sifat Salatun-Nabi, pg. 204.

Chapter: After the last tashahhud and before salam