The Book of Zuhd and Softening of Hearts
وَحَدَّثَنَا أَبُو كُرَيْبٍ، مُحَمَّدُ بْنُ الْعَلاَءِ حَدَّثَنَا وَكِيعٌ، عَنْ قُرَّةَ بْنِ خَالِدٍ، عَنْ حُمَيْدِ، بْنِ هِلاَلٍ عَنْ خَالِدِ بْنِ عُمَيْرٍ، قَالَ سَمِعْتُ عُتْبَةَ بْنَ غَزْوَانَ، يَقُولُ لَقَدْ رَأَيْتُنِي سَابِعَ سَبْعَةٍ مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم مَا طَعَامُنَا إِلاَّ وَرَقُ الْحُبْلَةِ حَتَّى قَرِحَتْ أَشْدَاقُنَا .Khalid b. Umair reported:I heard Uqba b. Ghazwan as saving: I found myself as the seventh amongst the seven who had been along with Allah's Messenger (ﷺ). We had nothing to eat but the leaves of hubla (a wild tree) until the corners of our mouths were injured
From Sahih Muslim, whose reports the scholars accept as authentic.