It is narrated from Hazrat Nafi that Sayyiduna Abdullah bin Umar (may Allah be pleased with them) would not wash his head in the state of Ihram, except when he experienced a wet dream.
Imam Malik, may Allah have mercy on him, said: I heard from the people of knowledge, they used to say: There is no harm if a man washes his head with sidr or potash, etc., after throwing pebbles at Jamrah al-Aqabah, before shaving his head, because when he has thrown the pebbles at Jamrah al-Aqabah, it becomes permissible for him to kill lice, shave his head, remove dirt, and wear clothes.
Hadith Referenceموطا امام مالك رواية يحييٰ / كتاب الحج / 710
Hadith Gradingمحدثین:موقوف صحيح
Hadith Takhrij«موقوف صحيح، و أخرجه البيهقي فى «معرفة السنن والآثار» برقم: 2873، والشافعي فى «الاُم » برقم: 252/7، فواد عبدالباقي نمبر: 20 - كِتَابُ الْحَجِّ-ح: 7»