Hazrat Mahdi (as) will have a light beard at the sides And long at the front

... [HAZRAT MAHDI (AS) IS] A MAN OF AVERAGE HEIGHT, DARK, WITH A LIGHT BEARD AT THE SIDES AND LONG IN THE FRONT; ... ONE WHO WILL BE KNOWN AS SHUAYB IBN SALEH. HE WILL HAVE WHITE CLOTHES, A BLACK BANNER AND 4000 PEOPLE UNDER HIS COMMAND... (Ibn Hajar al-Haythami, al-Fatawa al-Hadithiyya, p. 41) 

One of the details our Prophet (saas) supplies about Hazrat Mahdi (as) is to do with his beard. The Prophet (saas) has said in several hadiths that Hazrat Mahdi (as) will be bearded, and in this hadith he provides detail about the shape of his beard. He says that the texture of his beard will be light and thin at the sides while the lower part is long.

In this hadith the Prophet (saas) addresses Hazrat Mahdi (as) as Shuayb ibn Saleh. In interpreting this hadith, the great Islamic scholar al-Haythami said that the word Shuaybused for Hazrat Mahdi (as) means a small division, meaning that the number of Hazrat Mahdi’s (as) followers waging an intellectual struggle against unbelievers with him will be very low. He also explained that the term ibn Salehnotes that Hazrat Mahdi’s (as) father will also be a very righteous man. On the basis of the trustworthy hadiths about Hazrat Mahdi (as), who the Prophet (saas) says will wear white clothes, al-Haythami has provided a further piece of information, saying that Hazrat Mahdi (as) will also bear the name of al-Harith, meaning Lion.