There is a Hadeeth related by Al-Khatib Al-Baghdaadi from Jaabir, which states:
"The prophet SalAllaahu 'Aleyhi wa Sallem at the time he returned from battle said: 'We have all just returned to the best of places and you have returned from Jihaad Asghar (the lesser Jihaad) to strive in Jihaad Akbar (the greater Jihaad).'
The companions asked, 'What is Jihaad Akbar ya Rasul Allaah?' He answered: 'The Jihaad of someone against his desires.'"
— Tarikh al-Baghdaadi 13/493.
It turns out that this Hadeeth is weak because within its Sanad there is a narrator by the name of Khalaf bin Muhammad bin Ismail al Khiyam who, according to al-Hakim: "His Hadeeths are unreliable." And Abu Ya'la al-Khalili says: "He often adulterated, is very weak and narrates unknown Hadeeth."
Ibn Taymiyyah states: "There is a Hadeeth related by a group of people which states that the Prophet SalAllaahu 'Aleyhi wa Sallem said after the battle of Tabuk: 'We have returned from Jihaad Asghar to Jihaad Akbar.' This Hadeeth has no source, nobody whomsoever in the Islamic field of knowledge has narrated it. Jihaad against the disbelievers is the most noble of actions and moreover it is the most important action for the sake of mankind."