Assalamu Alaikum wa rahmatullahi wa barakatuhu. There is a hadith that says, if someone wakes up at night and says 'La ilalaha illaallahu wahdahu la sharika lahu...' and then ask forgiveness, he will be forgiven and if he supplicates, it will be answered and if he do wudhu and pray, it will be accepted from him. My question is about the session of waking up 'at night'. Is the 'night' means after 'Isha till the beginning of Fajr or after sunset till sunrise? or there is no specific time limit in other narrations of the same hadith?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

We have not come across any report that defines part of the night. The meaning of "the night" here may be understood as referring to the period between Al-ʿIshaa’ prayer and Al-Fajr prayer for the following two points:

1- It is detestable to sleep before Al-ʿIshaa’ prayer.

2- The observance of the night prayer starts from Al-ʿIshaa’ prayer to Al-Fajr prayer.

As for the cited Hadeeth, it is reported by Imam Al-Bukhari in the book on At-Tahajjud, specifically in the chapter on Faddlu Man Taʿarra Mena Al-Layl Fasala (the Excellence of the One Who Gets Up by Night and Prays). Hadeeth scholars usually cite this Hadeeth when talking about the night prayer.

Ibn Al-Mullaqqin said in At-Tawdheeh: “It is recommended for whoever knows about this Hadeeth to seize the opportunity and act upon it and to have sincere intention to his Lord, the Almighty, so that He would enable him to observe the night prayer.” [End of quote]

Allah knows best.