Shama-il Tirmidhi Chapter 001, Hadith Number 010 (010).
Abu-Ishaaq (Radiallahu anhu) says: "A person once asked Baraa bin Aazib (Radiallahu anhu), "Was the face of Rasulullah (Sallallahu alaihe wasallam) shining like a sword?" He replied: "No but like a full-moon with its roundness."
In making a comparison with a sword, it may have meant that Sayyidina Rasulullah (Sallallahu alaihe wasallam) had a long face. However the glitter of a sword has more whiteness than being luminous. For this reason Sayyidina Baraa (Radiallahu anhu) gave the similarity of the full moon, and not that of a sword. All these similarities are approximate descriptions, otherwise even a thousand moons cannot give the brightness of Sayyidina Rasulullah (Sallallahu alaihe wasallam). An Arab poet says: "If you want to describe a shortcoming of a beloved, then give the beloved a similitude of a full moon. This is enough to insult him."