Before commencing one’s tahajjud salaah, one should recite Takbeer (Allahu Akbar) ten times, Tahmeed (Alhamdulllah) ten times, Tasbeeh (SubhaanAllah) ten times, and Istighfaar (Astaghfirullah) ten times. Thereafter one should recite the following Duaa
اللَّهُمَّ اغْفِرْ لِي وَاهْدِنِي وَارْزُقْنِي وَعَافِنِي أَعُوذُ بِاللَّهِ مِنْ ضِيقِ الْمَقَامِ يَوْمَ الْقِيَامَةِ
O Allah, forgive my sins, guide me to the straight path, grant me sustenance and bless me with aafiyah (good health and good conditions). I seek the protection of Allah from the difficulty of the day of Qiyaamah.
عن عاصم بن حميد قال: سألت عائشة ماذا كان النبي صلى الله عليه و سلم يفتتح به قيام الليل ؟ قالت لقد سألتني عن شيء ما سألني عنه أحد قبلك . كان يكبر عشرا . ويحمد عشرا . ويسبح عشرا . ويستغفر عشرا . ويقول: اللهم اغفر لي واهدني وارزقني وعافني ) ويتعوذ من ضيق المقام يوم القيامة . (ابن ماجة رقم 1356، ابو داود رقم 766)
Hadhrat Aasim bin Humaid (Rahmatullahi Alaih) reports: “I once asked Hadhrat Aaisha (Radhiyallahu Anha) regarding the zikr which Rasulullah (Sallallahu Alaihi Wasallam) used to recite at the time of Tahajjud before commencing his Tahajjud salaah. Hadhrat Aaisha (Radhiyallahu Anha) replied: ‘You asked me regarding something that no one before you has asked me.’ She then said: ‘Rasulullah (Sallallahu Alaihi Wasallam) would recite Takbeer (Allahu Akbar) ten times, Tahmeed (Alhamdulllah) ten times, Tasbeeh (SubhaanAllah) ten times, and Istighfaar (Astaghfirullah) ten times. Thereafter he would recite the following Duaa and then seek protection in Allah Ta’ala from the difficulty of the day of Qiyaamah.
اللَّهُمَّ اغْفِرْ لِي وَاهْدِنِي وَارْزُقْنِي وَعَافِنِي
Source: Ihyaauddeen.co.za