Muaz said to us during his illness, "I heard something from the Messenger of Allah ﷺ that I had been keeping from you. I heard the Messenger of Allah ﷺ say, 'Whoever's last words are, There is no god but Allah, Paradise becomes guaranteed for him.'"
حدثنا محمد بن بكر أنا عبد الحميد يعني ابن جعفر حدثنا صالح يعني ابن أبي عريب عن كثير بن مرة
قال لنا معاذ في مرضه قد سمعت من رسول الله ﷺ شيئا كنت أكتمكموه سمعت رسول الله ﷺ يقول من كان آخر كلامه لا إله إلا الله وجبت له الجنة
Its isnad is sahih. Kathir bin Mura is trustworthy, his hadith is found frequently in the collections of the group, and he was from Hims, one of the companions of Muaz. Abu Dawud narrated the hadith, 3/187, no. 3116, and Hakim graded it sahih, 1/351, and Dhahabi agreed with him. It has already appeared with similar wording within hadith 10935.