Al-Matalib al-'Aliyah 253 — The prayer times

It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that he wrote to Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him): "Pray the Zuhr prayer when the sun has passed its zenith; pray the Asr prayer while the sun is still bright, white and clear in its original brilliance; pray the Maghrib prayer as soon as the sun sets; and pray the Isha prayer in the time between the disappearance of the twilight (redness) and the end of the first half of the night, for that is the Sunnah. Establish the dawn (Fajr) prayer while darkness still remains, and make the recitation in it long."
Imam al-Harith bin Abi Usamah recorded this report in his Musnad, and all its narrators are thiqat (trustworthy). This instruction is also narrated with reliable chains in other books of hadith, and it explains the preferred times of the prayers.