The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1872
Iyas b. Salama b. al-Akwa' reported on the authority of his father: We used to observe the Friday prayer with the Messenger of Allah (may peace be upon him) when the sun passed the meridian. and we then returned and tried to find out afternoon shadow (of the walls for protecting themselves from the heat of the sun).