Assalamualikumwarahmatullah to all, just need quick clarification on this hadith please
"I noticed that the Messenger of Allah (ﷺ) was missing from bed, so I started to look for him, and I thought that he had gone to one of his concubines. Then my hand fell on him when he was prostrating and saying: "Allahummaghfirli ma asrartu wa ma a'lant (O Allah, forgive me for what (sin) I have concealed and what I have done openly).'
reference : Sunan an-Nasa'i 1124
In-book reference : Book 12, Hadith 96
English translation : Vol. 2, Book 12, Hadith 1125
What is meant by the word "concubines" does it mean "wives" or something else, any assistance would be appreciated.