Sahih Muslim
The Book Pertaining to Piety and Softening of Hearts (Kitab Al-Zuhd wa Al-Raqaiq)
Book Number: 042 / Hadith Number: 7077
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 (may peace be upon him). We had nothing to eat but the leaves of hubla (a wild tree) until the corners of our mouths were injured.