From Tafsir Ibn Kathir: Surah 17 Al Isra’ Ayat 85
As-Suhayili mentioned the dispute among the scholars over whether the Ruh is the same as the Nafs, or something different. He stated that it is light and soft, like air, flowing through the body like water through the veins of a tree. He states that the Ruh which the angel breathes into the fetus is the Nafs, provided that it joins the body and acquires certain qualities because of it, whether good or bad.
So then it is either a soul in (complete) rest and satisfaction (89:27) or inclined to evil (12:53), just as water is the life of the tree, then by mixing with it, it produces something else, so that if it mixes with grapes and the grapes are then squeezed, it becomes juice or wine.
Then it is no longer called water, except in a metaphorical sense. Thus we should understand the connection between Nafs and Ruh; the Ruh is not called Nafs except when it joins the body and is affected by it.
So in conclusion we may say: the Ruh is the origin and essence, and the Nafs consists of the Ruh and its connection to the body. So they are the same in one sense but not in another. This is a good explanation, and Allah knows best.
I say: people speak about the essence of the Ruh and its rulings, and many books have been written on this topic. One of the best of those who spoke of this was Al-Hafiz Ibn Mandah in a book which we have heard about the Ruh.
(86. And if We willed, We could surely take away that which We have revealed to you. Then you would find no protector for you against Us in that respect.)
(87. Except as a mercy from your Lord. Verily, His grace unto you is ever great.)
(88. Say: “If mankind and the Jinn were together to produce the like of this Qur’an, they could not produce the like thereof, even if they helped one another.”)
(89. And indeed We have fully explained to mankind, in this Qur’an, every kind of similitude, but most of mankind refuse (the truth and accept nothing) but disbelief.)