In his final illness the Prophet ﷺ asked for forgiveness, mercy and the company of the highest companions. Gently remind the dying of La ilaha illa-llah without pressing them, for the one whose last words they are will enter Paradise (Sunan Abi Dawud 3116).
Allahumma-ghfir li, warhamni, wa alhiqni bi-r-rafiqi-l-a'la
“O Allah, forgive me, have mercy on me, and join me with the highest companions.”
Among the last words of the Prophet ﷺ, as 'A'ishah heard them.
La ilaha illa-llahu wa-llahu akbar, la ilaha illa-llahu wahdah, la ilaha illa-llahu wahdahu la sharika lah, la ilaha illa-llahu lahu-l-mulku wa lahu-l-hamd, la ilaha illa-llahu wa la hawla wa la quwwata illa billah
“There is no god but Allah, and Allah is the Greatest. There is no god but Allah alone. There is no god but Allah alone, with no partner. There is no god but Allah; His is the dominion and His is the praise. There is no god but Allah, and there is no power and no strength except through Allah.”
The Prophet ﷺ said that Allah (SWT) affirms each of these words, and that whoever says them in his illness and then dies, the Fire will not consume him. Al-Albani graded it sahih; Darussalam's checkers of at-Tirmidhi and Ibn Majah graded it weak.