بُلُوغُ الْمَرَامِ
Bulugh al-Maram 466
The Book of Prayer
وَجَابِرِ عِنْدَ أَبِي دَاوُدَ, وَالنَّسَائِيّ ِ : [1] { أَنَّهَا مَا بَيْنَ صَلَاةِ اَلْعَصْرِ إِلَى غُرُوبِ اَلشَّمْسِ } ِ.
and Abu Dawud and an-Nasa'i reported from Jabir: "It is between the 'Asr prayer and sunset."
Sunnah.com reference: Book 2, Hadith 387