The Prophet (ﷺ) used to stand in the night prayer and say this first 10 times:
اَللَّهُ اَكْبَرُ
اَللّٰهُ اَكْبَرُ
Allahu Akbaar
Allah is the greatest!
10 times:
سُبْحَانَ اللَّهِ
سُبْحَانَ اللّٰهِ
SubhaanAllah
Glory is to Allah.
10 times:
اَسْتَغْفِرُ اللَّهَ
اَسْتَغْفِرُ اللّٰهَ
astaghfir-ullaah
I seek Allah’s forgiveness.
then he would say:
اَللَّهُمَّ اغْفِرْ لِي وَاهْدِنِي، وَارْزُقْنِي وَعَافِنِي
اَللّٰهُمَّ اغْفِرْ لِيْ وَاهْدِنِيْ، وَارْزُقْنِيْ وَعَافِنِيْ
Allaahumma-ghfir lee wah-dinee, war-zuqnee wa ‛aafinee
O Allah, forgive me and guide me [to righteousness], provide me subsistence and grant my body health.
and also he would say:
أَعُوْذُ بِاللَّهِ مِنْ ضِيْقِ الْمَقَامِ يَوْمَ الْقِيَامَةِ
اَعُوْذُ بِاللّٰهِ مِنْ ضِيْقِ الْمَقَامِ يَوْمَ الْقِيَامَةِ
A‛oodhu billaahi min deeqil-maqaami yawmal-qiyaamah
I seek refuge in Allah from the tightness (or suffocation) of my standing on the day of judgement.
Asim ibn Humayd said: I asked Aisha (RA): By what words the Messenger of Allah (ﷺ) would begin his tahajjud prayer at night? She replied: You ask me about a thing of which no one asked me before you. (Then she mentioned the above dua)