Sahih Bukhari | Volume 3 | Hadith Number 41

Narrated by 'Abdur-Rahman bin Abu Layla
Ka'b bin 'Ujra said that Allah's Apostle said to him (Ka'b), "Perhaps your lice have troubled you?" Ka'b replied, "Yes! O Allah's Apostle." Allah's Apostle said, "Have your head shaved and then either fast three days or feed six poor persons or slaughter one sheep as a sacrifice."