Wearing a turban is mustahab (desirable) and the Holy Prophet (Sallallahu Alaihi Wa Sallam) has also commanded to do so. “Keep to turbans. This will make you more forbearing (Fathul Bari). When asked by someone whether wearing a turban is a Sunnah Hadrat Abdullah lbn Umar (Radi Allaahu Ta’ala Anhu) replied in the affirmative (Aameen).
Amr bin – Hurais (Radi Allaahu Ta’ala Anhu) said, “I still have that scene before my eyes when the Holy Prophet (Sallallahu Alaihi Wa Sa.Ilam) delivered a sermon from the member (pulpit) and that he was wearing a black turban with its end hanging between the two shoulders.” [Khasael-e-Nabawi, from Muslims and Nasa]
When he used to put on a turban, he used to lower down the end between the two shoulders and often his turban did not have a loose end. [Nashrut-Tib, Shamail-e-Tirmizi]
The loose end of his turban was about a span in length. But a length of more than one cubit has also been reported while the turban itself used to be about seven yards in length.[Khasail-e-Nabawi]
To have a cap underneath the turban is Sunnah.