Uyaynah bin Abd al-Rahman bin Jawshan narrates from his father that a man said to Sayyiduna Abdullah bin Abbas (may Allah be pleased with him): May I scratch my head while I am in the state of ihram? He said: Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) put his hand into his hair while he was in ihram and scratched his head vigorously, and said: As for me, I do it like this. He said: What do you think if I kill a louse? He said: You have gone far off! What is a louse? It makes no difference to the scratching of your head, it was not the louse you intended, and you have been forbidden only from killing game.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10064