Sahih al-Bukhari Book 9, Hadith 32
Allah's Messenger (ﷺ) said, "Angels come to you in succession by night and day and all of them get together
at the time of the Fajr and `Asr prayers. Those who have passed the night with you (or stayed with
you) ascend (to the Heaven) and Allah asks them, though He knows everything about you, well, "In
what state did you leave my slaves?" The angels reply: "When we left them they were praying and
when we reached them, they were praying."
Jarir said, "We were with the Prophet (ﷺ) 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 sunrise (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."
Allah's Messenger (ﷺ) said, "If anyone of you can get one rak`a of the `Asr prayer before sunset, he should complete his prayer. If any of you can get one rak`a of the Fajr prayer before sunrise, he should complete his prayer."