كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(34)
Chapter: It is recommended to pray `Asr early
(34)
باب اسْتِحْبَابِ التَّبْكِيرِ بِالْعَصْرِ
Sahih Muslim 621 c

Anas b. Malik reported:

We used to offer the 'Asr prayer, then one would go to Quba' and reach there and the sun would be still high.
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كُنَّا نُصَلِّي الْعَصْرَ ثُمَّ يَذْهَبُ الذَّاهِبُ إِلَى قُبَاءٍ فَيَأْتِيهِمْ وَالشَّمْسُ مُرْتَفِعَةٌ ‏.‏
Reference : Sahih Muslim 621c
In-book reference : Book 5, Hadith 244
USC-MSA web (English) reference : Book 4, Hadith 1299
  (deprecated numbering scheme)