Sahih Al Bukhari Hadith Number 1814

Narrated `Abdur-Rahman bin Abu Layla: Ka`b bin ‘Ujra said that Allah’s Messenger (peace be upon him) said to him (Ka`b), “Perhaps your lice have troubled you?” Ka`b replied, “Yes! O Allah’s Messenger (peace be upon him).” Allah’s Messenger (peace be upon him) said, “Have your head shaved and then either fast three days or feed six poor persons or slaughter one sheep as a sacrifice.”

27  Pilgrims Prevented from Completing the Pilgrimage
Hadith number 1814
Volume no 3
Chapter: 5 (The Statement of Allah Ta’ala : “… And whosoever of you is ill or has an ailment in his scalp, (necessitating shaving), he must pay a Fidya …”)