Musannaf Ibn Abi Shaybah

Hazrat Zuhri narrates that Hazrat Abu Salamah bin Abdur Rahman, when he would have lawful relations with his wife, disliked that people should come to know of it. Therefore, he would perform ghusl up to his throat, but did not like to wash his head, lest people would guess by seeing his wet hair. Then he would go to his family and say, "I have a headache," and would have a plant called khatmi brought and would wash his head with it.