It is well known that he died saying 'Al salah, al salah!'
I couldn't find the al salah bit.
Abu Dawood (5156)and Ibn Maajah (2698) narrated that ?Ali (may Allaah be pleased with him) said: The last words that the Messenger of Allaah (peace and blessings of Allaah be upon him) spoke were, ?The prayer, the prayer! And fear Allaah with regard to those whom your right hands possess.? This was classed as saheeh by al-Albaani in Saheeh Abi Dawood. And there are other similar ahaadeeth.
Thanks, but Sahih Bukhari is the most authentic collection of hadith, ranked in importance right after the Quran & Sahih Bukhari hadiths say this quite unambiguously:
Volume 7, Book 62, Number 144:
Narrated 'Aisha:
that during his fatal ailment, Allah's Apostle, used to ask his wives, "Where shall I stay tomorrow? Where shall I stay tomorrow?" He was looking forward to Aisha's turn. So all his wives allowed him to stay where he wished, and he stayed at 'Aisha's house till he died there. 'Aisha added:
He died on the day of my usual turn at my house. Allah took him unto Him while his head was between my chest and my neck and his saliva was mixed with my saliva. I don't think that Ali was a spectator in the bedroom where Ayesha & Muhammad had shared an intimate moment-so those might well be Muhammad's last words in public, it doesn't mean that he died reciting those, as fading said.
Aisha who in all probability was the sole spectator, recalls no such words & the position Muhammad was in ie "while his head was between my chest and my neck and his saliva was mixed with my saliva" would make it difficult to speak anything at all.
Was Muhammad mumbling those words with his head between Ayesha's chest & neck & peeping Tom Ali heard those?

So he didn't say those words while dying...