Reference : Hisn al-Muslim 61

اللّهُـمَّ إِنِّـي أَسْأَلُـكَ الجَـنَّةَ 
وأََعوذُ بِـكَ مِـنَ الـنّار
Allāhumma innī as'aluka ‘l-jannah
wa a`ūdhu bika minan-nār.


O Allah, I ask You for Paradise,
and I seek Your protection from the Fire.

Reference:
Abu Dawud. See also Al-Albani, Sahih Ibn Majah 2/328.

Chapter: After the last tashahhud and before salam