It is narrated from Hazrat Abu Saeed Khudri (may Allah be pleased with him) that Hazrat Qatadah bin Nu’man (may Allah be pleased with him) once spent the whole night reciting only Surah Ikhlas. When this was mentioned before the Prophet (peace and blessings be upon him), he said, "By Him in Whose hand is my soul! Surah Ikhlas is equal to half or one third of the Qur’an."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 11115
Hadith Gradingحکم دارالسلام:إسناده ضعيف لضعف ابن لهيعة، والمحفوظ الثابت الصحيح فى هذا الحديث وغيره: انها تعدل ثلث القرآن دون شك