When the rim of the sun starts appearing defer prayer till it completely appears, and when the rim of the sun disappears defer prayer till it completely disappears.
Sahih Muslim Book 6, Hadith 354
← Previous Hadith Do not intend to observe prayer at the time of the rising of the sun nor at its setting, for it rises between the horns of Satan.

Sahih Muslim Book 6, Hadith 353 Next Hadith → 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).

Sahih Muslim Book 6, Hadith 355