From Sahih Muslim (Hadith 1927):
Abu Basra Ghifari reported:The Messenger of Allah (ﷺ) led us in the 'Asr prayer at (the place known as) Mukhammas, and then said: This prayer was presented to those gone before you, but they lost it, and he who guards it has two rewards in store for him. And no prayer is valid after till the onlooker appears (by onlooker is meant the evening star)
Reference: Muslim 1927