It is narrated from Hazrat Abu Huraira (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) heard a man reciting Surah Ikhlas, so he said, "It has become obligatory." The people asked, "O Messenger of Allah! What has become obligatory?" He replied, "Paradise has become obligatory for him."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 8011