Invocation
After the last tashahhud and before salam
اللّهُـمَّ إِنِّـي أَسْأَلُـكَ بِأَنَّـي أَشْـهَدُ أَنَّـكَ أنْـتَ اللهُ لا إِلـهَ إِلاّ أَنْـت الأَحَـدُ الصَّـمَدُ الَّـذي لَـمْ يَلِـدْ وَلَمْ يولَـدْ وَلَمْ يَكـنْ لَهُ كُـفُواً أَحَـد
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.
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.