Reference : Hisn al-Muslim 65

اللّهُـمَّ إِنِّـي أَسْأَلُـكَ 
بِأَنَّـي أَشْـهَدُ أَنَّـكَ أنْـتَ اللهُ 
لا إِلـهَ إِلاّ أَنْـت
الأَحَـدُ الصَّـمَدُ 
الَّـذي لَـمْ يَلِـدْ وَلَمْ يولَـدْ
وَلَمْ يَكـنْ لَهُ كُـفُواً أَحَـد
Allāhumma innī as'aluka 
bi'annī ash-hadu annaka ant-Allāh 
lā 'ilāha 'illā ant,
Al-Aḥaduṣ-ṣamad
alladhī lam yalid wa lam yūlad 
wa lam yakun lahu kufuwan aḥad.


O Allah, I ask You, by the fact that I bear witness that You are Allah. 
There is none worthy of worship but You, 
the Only God, Independent of creation, 
Who was not begotten and begets not, 
and none is equal to Him.

Reference:
Abu Dawud 2/62, Ibn Majah 2/1267, At-Tirmidhi 5/515, Ahmad 5/360. See also 
Al-Albani, Sahih Ibn Majah 2/329 and Sahih At-Tirmidhi 3/163.

Chapter: After the last tashahhud and before salam