Dua #1022•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.

After the last tashahhud and before salam

Hisnul Muslim
#1022

للّهُـمَّ إِنِّـي أَسْأَلُـكَ بِأَنَّـي أَشْـهَدُ أَنَّـكَ أنْـتَ اللهُ لا إِلـهَ إِلاّ أَنْـت الأَحَـدُ الصَّـمَدُ الَّـذي لَـمْ يَلِـدْ وَلَمْ يولَـدْ وَلَمْ يَكـنْ لَهُ كُـفُواً أَحَـد

Transliteration

Allāhumma innī as'alukabi'annī ash-hadu annaka ant-Allāhlā 'ilāha 'illā ant. Al-Aḥaduṣ-ṣamadalladhī lam yalid wa lam yūladwa lam yakun lahu kufuwan aḥad.

Translation

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.