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.