´It was narrated that Aishah said:` "The Messenger of Allah (ﷺ) used to say when bowing: 'Subbuhun Quddusun Rabbul-mala'ikati war-ruh (Perfect, Most Holy, is the Lord of the angels and the spirit).'"
Related hadith on this topic
Explanation & Benefits
Hafiz Muhammad Ameen
1049. Commentary: What is meant by “the Spirit”? It is said that it refers to Jibril (alayhis salam), or a creation higher than the angels who sees the angels while the angels do not see it, or the human souls. However, the Noble Qur’an explicitly clarifies that what is meant by it is indeed Jibril al-Amin (the Trustworthy), and due to his honor and rank, he is mentioned separately after the angels. Allah the Exalted says: “The Trustworthy Spirit has brought it down” [الشعراء 193 : 26] — “This (Qur’an) has been brought down by the Trustworthy Angel.”
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1049
Hafiz Muhammad Ameen
1135. Commentary: For benefits, see Hadith number: 1049.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1135
Shaykh Maulana Abdul Aziz Alvi
Hadith Commentary: Benefits and Issues: By "the Spirit" (ruh) is meant Jibril (Gabriel) alayhis salam. Some have said that this refers to another great angel or an independent creation, whom even the angels cannot see.
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 1091