I came across a brother who said to me that a man’s prayer is not accepted if he is not wearing a topi or head covering?
Answer:
In the name of Allah, the most Beneficent, the most Merciful.
It is desirable and Mustuhub to cover one’s head whilst praying Salah. To omit it out of laziness is considered to be Makruh. (Raddul Muhtar p.474 v.1)
Hasan Basri 
says, “The Companions would make Sajdah during Salah on their turbans and hats.” (Sahih Bukhari p.56 v.1)
Saaiduna Rukanah 
narrates, “I heard the Prophet of Allah 
saying, ‘Verily, the distinguishing feature between us and the Mushrikeen is that we wear the turban over the topi’.” (Sunan Tirmizi p.308 v.1)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham