The caps of the companions of the Messenger of Allah ﷺ were flat-topped.
حدثنا حميد بن مسعدة حدثنا محمد بن حمران عن أبي سعيد وهو عبد الله بن بسر قال سمعت
كانت كمام أصحاب رسول الله ﷺ بطحا
No references are available.
Sourcing, its chain of transmission is daif. Abu Shaykh also recorded it in Akhlaq an-Nabi, p. 102, from the hadith of Muhammad bin Humran, with different wordings.