I came along with the Prophet ﷺ when he heard a man reciting, "Say, He is Allah, the One. Allah, the Eternal Refuge."The Messenger of Allah ﷺ said, "It has become obligatory."I asked, "What has become obligatory?"He replied, "Paradise."
حدثنا أبو كريب حدثنا إسحق بن سليمان عن مالك بن أنس عن عبيد الله بن عبد الرحمن عن ابن حنين مولى لآل زيد بن الخطاب أو مولى زيد بن الخطاب
أقبلت مع النبي ﷺ فسمع رجلا يقرأ قل هو الله أحد الله الصمد فقال رسول الله ﷺ وجبت قلت وما وجبت قال الجنة
Its isnad is hasan. Nasai narrated it: 2/171, no. 995, Book of the Opening of Prayer, chapter on the merit of reciting "Say, He is Allah, the One," from the hadith of Malik through the same route, and it is found in Muwatta, 1/208. Hakim graded it sahih, 1/566, and Dhahabi agreed with him.