الجامع المختصر لفضائل الآيات والسور
Virtues of the Qur'an's Chapters and Verses
The Messenger would pray two rakʿahs after witr with this sūrah
كَانَ الرَّسُولُ ﷺ يُصَلِّي رَكْعَتَيْنِ بَعْدَ الْوِتْرِ بهَذِهِ السُّورَةِ
Virtues of the Qur'an's Chapters and Verses 67
The Messenger of Allah ﷺ would pray two rakʿahs after witr while sitting, reciting in the two rakʿahs “Idhā zulzilat al-ardhu…” (Sūrah al-Zalzalah) and “Qul Ya Ayyuhal-Kafirun” (Sūrat al-Kāfirūn). Reference: Zad al-Ma`ad 1/322

Is Sūrat al-Zalzalah equivalent to one-fourth of the Qur’an?
The reports about Sūrat Al-Zalzalah being equivalent to one-fourth of the Qur’an have some weakness in them, although they are strengthened by the different variations as well as the multiple mursal reports (which are weak standalone). Ibn Khuzaymah categorized one particular attributed to Anas (RA) as saḥiḥ and al-Tirmidhī classified one narration as ḥasan, while others relied on the report of Ibn Abbās.

كان رسولُ اللهِ صلَّى اللهُ عليه وسلم يصلِّي ركعتينِ بعد الوِترِ وهو جالسٌ يقرأُ فيهما ب { إِذَا زُلْزِلَتِ الْأَرْضُ . . . ) و{ قُلْ يَا أَيُّهَا الْكَافِرُونَ ).
Reference : Virtues of the Qur'an's Chapters and Verses 67