Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 356
Narated By N/A : Uthaim b. Kulaib from his father (Kuthair) on the authority of his grandfather (Kulaib) that he came to the Prophet (pbuh): I have embraced Islam. The Prophet (pbuh) said to him: Remove from yourself the hair that grew during the period of unbelief, saying "shave them".
He further says that another person (other than the grandfather of 'Uthaim) reported to him that the Prophet (pbuh) said to another person who accompanied him: Remove from yourself the hair that grew during the period of unbeleif and get yourself circumcised.