How authentic is the Hadith: "The Mujahid is he who strives against his own self in obedience to Allah"?

Imam Ahmad and others narrated from Fudalah ibn 'Ubayd that the Prophet, peace and blessings be upon him, said during the Farewell Pilgrimage: "The believer is he from whom people feel secure regarding their wealth and lives, and the Muslim is he from whose tongue and hand people are safe, and the Mujahid (struggler) is he who strives against his own self in obedience to Allah, and the Muhajir (emigrant) is he who abandons sins and transgressions." This hadith (definition) was authenticated by Al-Tirmidhi, Al-Hakim, and Al-Albani. Sheikh al-Islam Ibn Taymiyyah explained that striving against the self (jihad an-nafs) is more necessary than striving against the enemy because it is an individual obligation (fard 'ayn), whereas striving against the enemy is a collective obligation (fard kifayah). He who perseveres in striving against his self will then persevere in the other striving. Ibn al-Qayyim said that striving against the self takes precedence over striving against the external enemy and is its foundation, for striving against the enemy is not possible except after striving against the self. Ibn Rajab clarified that striving against the self in obedience to Allah is the greatest struggle, and it requires patience. He who is patient will prevail, and he who is impatient will be overcome.