Sahih Bukhari, Chapter: Times of the Prayers

Jarir said, "We were with the Prophet and he looked at the moon--full-moon--and said, 'Certainly you will see your Lord as you see this moon and you will have no trouble in seeing Him. So if you can avoid missing (through sleep or business, etc.) a prayer before the sun-rise (Fajr) and a prayer before sunset ('Asr), you must do so.' He then recited Allah's Statement: And celebrate the praises Of your Lord before The rising of the sun And before (its) setting." (50.39) Isma'il said, "Offer those prayers and do not miss them."