Sa'id bin Abi Sa'id (may Allah have mercy on him) narrates from his father that he saw Sayyiduna Abu Rafi' (may Allah be pleased with him), the freed slave of the Messenger of Allah (peace and blessings be upon him), pass by Sayyiduna Husayn bin Ali (may Allah be pleased with them both) while Husayn was praying standing, and he had tucked his braid into the nape of his neck. Abu Rafi' (may Allah be pleased with him) undid it, and Husayn (may Allah be pleased with him) turned around angrily. Abu Rafi' (may Allah be pleased with him) said: Attend to your prayer and do not be angry, for I heard the Messenger of Allah (peace and blessings be upon him) saying: "That is the seat of Satan." That is, the place where Satan sits, meaning the place where the braid is tucked.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 989