Sunan Abu Dawood, Chapter: Prayer (Kitab Al-Salat)

The Apostle of Allah (peace_be_upon_him) taught me and what he taught me is this: Observe the five prayers regularly. He said: I told (him): I have many works at these times; so give me a comprehensive advice which, if I follow, should be enough for me. He said: Observe the two afternoon prayers (al-asrayn). But the term al-asrayn (two afternoon prayers) was not used in our language. Hence I said: What is al-asrayn? He said: A prayer before the sunrise and a prayer before the sunset (i.e. the dawn and the afternoon prayers).