Q. I was reading Tahajjud Salah and the time of Fajr Salah entered. Did my Tahajjud Salah break and do I have to repeat it?
A. The time for performing Tahajjud Salaah begins after Esha Salaah and extends up to Subah Saadiq (true dawn), although it is more virtuous to perform Tahajjud Salaah in the latter portion of the night.
In the enquired case, if a person commenced Tahajjud Salaah before Subah Saadiq ends and completed it after Subah Saadiq had already ended, the Salaah is considered complete. Therefore, there is no need to repeat it. (Shaami 1/374)
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa