Ata (may Allah have mercy on him) said: For one hair there is (the penalty of) one mudd, for two hairs two mudds, and for three or more, the shedding of blood (a sacrifice).
It is narrated from al-Hasan al-Basri and Ata (may Allah have mercy on them) that they said: For three hairs there is the shedding of blood; the one who forgets and the one who does it deliberately are equal in this.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9124