Sunan an-Nasa'i, Volume 4, Book 25, Hadith 3131 (DarusSalam) ⓘ
It was narrated from Abu Dharr that he asked the prophet of Allah which deed was best. He said: "Belief in Allah and Jihad in the cause of Allah, the Mighty and Sublime."
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ عَبْدِ الْحَكَمِ، عَنْ شُعَيْبٍ، عَنِ اللَّيْثِ، عَنْ عُبَيْدِ اللَّهِ بْنِ أَبِي جَعْفَرٍ، قَالَ أَخْبَرَنِي عُرْوَةُ، عَنْ أَبِي مُرَاوِحٍ، عَنْ أَبِي ذَرٍّ، أَنَّهُ سَأَلَ نَبِيَّ اللَّهِ صلى الله عليه وسلم أَىُّ الْعَمَلِ خَيْرٌ قَالَ " إِيمَانٌ بِاللَّهِ وَجِهَادٌ فِي سَبِيلِ اللَّهِ عَزَّ وَجَلَّ " .
also referenced under: Hadith 3129 (Reference) & Book 25, Hadith 45 (In-Book)