Musannaf Ibn Abi Shayba 1:770

Narrated to us Abd al-Rahim, from Ismail, from al-Zuhri, who said: Abu Salamah ibn Abd al-Rahman used to conceal himself from his family, and he disliked that they should know about him. He would wash his body up to his neck, and he disliked washing his head so that they would not know about it. So he would come to his family and say, "I feel something in my head," then he would call for *khatmi* (marshmallow plant) and wash it.