O Allah, do not make this the last prayer I ever perform. — After Prayer Adhkar
اللَّهُمَّ لَا تَجْعَلْهَا آخِرَ صَلَاةٍ صَلَّيْتُهَا
Transliteration: Allaahumma laa taj'alhaa aakhira salaatin sallaytuhaa
Translation: O Allah, do not make this the last prayer I ever perform.
Reference: General supplication