The Messenger of Allah (ﷺ) used to say towards the end of Witr prayer:

اَللَّهُمَّ إِنِّي أَعُوْذُ بِرِضَاكَ مِنْ سَخَطِكَ، وَبِمُعَافَاتِكَ مِنْ عُقُوبَتِكَ، وَأَعُــوذُ بِكَ مِنْكَ، لَا أُحْصِيْ ثَنَاءً عَلَيْكَ، أَنْتَ كَمَا أَثْنَيْتَ عَلَى نَفْسِكَ

Allaahumma 'innee 'a'oodhu biridhaaka min sakhatik, wa bimu'aafaatika min 'uqoobatik wa 'a'oodhu bika minka, laa 'uhsee thanaa'an 'alayka 'Anta kamaa 'athnayta 'alaa nafsik

O Allah, I seek refuge with Your Pleasure from Your anger. I seek refuge in Your forgiveness from Your punishment. I seek refuge in You from You. I cannot count Your praises, You are as You have praised Yourself. Ali (RA) said, Rasulullah (ﷺ) used to say the words at the end of Witr prayer. The end of Witr prayer here is not clearly stated before the salaam or after the salaam. Therefore, a believer can recite this dua at the end of Salatul Witr prayer before Salaam or after Salaam.

Reference: Sahih. Abu Dawud: 1427