It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "This prayer — that is, Asr — was made obligatory upon those before you, and they neglected it. So whoever among you today guards it will be given its reward twice; and there is no (voluntary) prayer after it until the witness — that is, the star — is seen."
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4069