The Book of Jihad and Expeditions
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا حَاتِمٌ، بِهَذَا الإِسْنَادِ . غَيْرَ أَنَّهُ قَالَ فِي كِلْتَيْهِمَا سَبْعَ غَزَوَاتٍ .The (foregoing) tradition has also been narrated on the authority of Hatim through the same chain of transmitters with the difference that according to this version both these types of expeditions were seven in number
From Sahih Muslim, whose reports the scholars accept as authentic.