Abu Dawud Book 27, Hadith Number 4072.
Chapter : Covering the head with a cloth.
Narated By N/A : ‘Aisha said: We were seated in our house in the noonday heat. Someone said to Abu Bakr: Here is the Apostle of Allah (peace be upon him) coming to us shading his head at the hour when he would not generally come. The Apostle of Allah (peace be upon him) then came; he asked for permission and he gave him permission and he entered.