Tahajjud

Can Tahajjud become fardh on a person?

Q: My question is about tahajjud...

My mother often, not regularly, wakes up for tahajjud and offers tahajjud namaaz as she cannot do so due to her health. She offers it regularly in Ramadhaan, Shawwal, Muharram and Zil Hajj, etc. (special months). Me and my sister also offer tahajjud daily Alhamdulillah.

Recently, one of my relatives told my mother that if you have started offering tahajjud namaaz or if you offered it one day, you cannot quit it. It becomes fardh for you. I told my mother that this is baseless since a nafal namaz cannot become fardh but she wants to confirm it. Please clarify this according to the Ahle-e-sunnat sect.