Sadly this practice is becoming more uncommon, especially among the younger generation. Hazrat Hasan Basri (Rahimahullah) narrates, “The Sahabah y used to perform Sajdah (prostrate) with turbans and topis.” (Bukhari)

Rasulullah (Sallallahu Alaihi Wasallam) said that a Muhrim (a person in Ihram) should not wear a kurta, turban, pants and a burnus (specific type of topi). (Bukhari)

We thus understand from this injunction that the Sahabah y were in the habit of wearing the topi in the time of Rasulullah r. Hence, the Muhrim is prevented from covering the head in the state of Ihram. This Hadith has been mentioned under the chapter of Burnus (head-cover).

Importantly, the topi is a distinguishing characteristic of a Muslim that serves as a mark of identity and respect.