I embraced Islam for Allah's sake. and in the hadith narrated by Ma'mar the words are: I knelt down to kill him, that he said; There is no god but Allah.
Sahih Muslim Book 1, Hadith 181
← Previous Hadith If I encountered a person amongst the infidels (in the battlefield) and he attacked me and struck me and cut off one of my hands with the sword. Then he (in order to protect himself from me) took shelter of a tree and said: I become Muslim for Allah's sake. Messenger of Allah, can I kill him after he had uttered this? The Messenger of Allah (ﷺ) said: Do not kill him. I (the narrator) said: Messenger of Allah, he cut off my hand and uttered this after amputating it; should I then kill him? The Messenger of Allah (ﷺ) said: Don't kill him, for I you kill him, verily he would be in a position where you had been before killing him and verily you would be in a position where he had been before uttering (kalima).

Sahih Muslim Book 1, Hadith 180 Next Hadith → Messenger of Allah, here is a point: If I happened to encounter a person amongst the infidels (in the battle). Then he narrated a hadith similar to the one transmitted by Laith.

Sahih Muslim Book 1, Hadith 182