Why did Prophet Muhammad(PBUH) called Jihad-al-Nafs the greatest jihad. Is it because its the most difficult struggle?
For you let me quote this
Al Baqarah, Chapter 2, 216
Fighting has been enjoined upon you while it is hateful to you. But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not.
Jihad Bil Saif is the greatest Jihad. If you read this Ayat 186 or 184
O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may become righteous.
If you see the Arabic words you will find there is no difference. Siyam and Jihad are equally obligatory (Farz).
Moreover, If you read the character of Our Holy Prophet SAW you will find he has fought 27 Battles and sent around 56 times the Army of Sahaba to fight against Kuffars in only 10 years meaning 1 battle in each 1 and half month. Still people quote what you are quoting. Although the ayat says already the most that you hate to fight because of the fear of death. So jihad in Battle field is the greatest Jihad which Quran says here and there is no authenticity about the quote you quoted. Also it conflicts the too many quotes of The Holy Quran and the life of Muhammad SAW and Sahaba r.a. For more details if you can read urdu read this authentic tafseer which is collection of old tafaseer about ayat e jihad.