Sayyiduna Ali (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "No prayer should be offered after Asr except when the sun is white (bright) and high."
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأوقات التى ينهى عن صلاة التطوع فيهن / 1284