Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. There are many Ahadith (sayings of the Prophet, peace be upon him) narrated in the Sahihain and other sources that describe the Prophet's way of smiling. According to these sources, the Prophet (Sallallahu Alaihi wa Sallam) used to smile in a manner that his front teeth would appear. In 'Zad Al-Ma'ad', Ibn Al-Qayyim (may Allah have mercy on him) said: Most of his laughter, actually all his laughter, for that matter, was to merely smile, in a way showing his front teeth. He smiled at whatever caused laughter or surprise. At-Tabarani in his 'al-Mu'jam al-Kabeer' and Al-Baihaqi in his 'Shu'ab al-Eman', narrated about the description of the Prophet (Sallallahu Alaihi wa Sallam) that: Most of his laughter was smiling. [Ruled as authentic by Imam Ibn Hajar in his 'Fath al-Bari'] For more details, please read Fatwa: 83159. Allah knows best.