It is narrated from Uthman bin Abd al-Rahman that Sayyiduna Anas bin Malik (may Allah be pleased with him) informed him that the Messenger of Allah (peace and blessings be upon him) used to offer the Asr prayer roughly so long before sunset that in that time a camel could be slaughtered, then we could cut it into portions, and so long before sunset that a man could go to Banu Harithah bin al-Harith and come back before the sun set.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2086