The Messenger of Allah ﷺ heard a man reciting "Say, He is Allah, the One" [Ikhlas 112:1] until he completed the surah. The Messenger of Allah ﷺ said, "It has become obligatory."Someone asked, "Messenger of Allah, what has become obligatory?"He answered, "Paradise."Abu Hurairah said, "I wanted to go to the man and give him the good news, but I preferred to have the morning meal with the Messenger of Allah ﷺ and feared missing it. Then I went back to find the man, but he had already left."
حدثنا عثمان بن عمر أخبرنا مالك عن عبد الله بن عبد الرحمن أن ابن حنين أخبره
أن رسول الله ﷺ سمع رجلا يقرأ قل هو الله أحد حتى ختمها فقال وجبت قيل يا رسول الله ما وجبت قال الجنة قال أبو هريرة فأردت أن آتيه فأبشره فآثرت الغداء مع رسول الله ﷺ وفرقت أن يفوتني الغداء مع رسول الله ﷺ ثم رجعت إلى الرجل فوجدته قد ذهب
Its isnad is sahih. Malik narrated it in 1/208, no. 18, in the book of Quran, chapter on what has been mentioned about Surah Ikhlas, and Tirmidhi narrated something similar in 5/168, no. 2897, and said, hasan gharib. Ibn Hunain is Ibrahim bin Abdullah bin Hunain, a trustworthy narrator.