The Messenger of Allah ﷺ said, "Surah az-Zalzalah equals half of the Quran, Surah al-Ikhlas equals a third of the Quran, and Surah al-Kafirun equals a quarter of the Quran."
حدثنا علي بن حجر أخبرنا يزيد بن هارون أخبرنا يمان بن المغيرة العنزي حدثنا عطاء
قال رسول الله ﷺ إذا زلزلت تعدل نصف القرآن وقل هو الله أحد تعدل ثلث القرآن وقل يا أيها الكافرون تعدل ربع القرآن
Reference, [its chain of transmission is daif]. Hakim narrated it in 1/566 through Yazid bin Harun with the same wording, and graded it sahih, but Dhahabi contested this, saying, "Rather, it is Yaman bin Mughira, they weakened him, and he is daif as stated in Taqrib and elsewhere."