Al-Matalib al-'Aliyah 251 — The prayer times

It is narrated from Sayyiduna al-Harith bin Amr al-Hudhali (may Allah be pleased with him) that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) sent a letter to Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him): "I have written to you about the prayer, and the thing most deserving of care and attention for the Muslims is the matter of their religion. I saw the Prophet (peace and blessings be upon him) praying; of that I remember what I was able to remember, and I have forgotten what I (in human fashion) forgot. So pray the Zuhr prayer in the intense midday heat, when the sun has passed its zenith; pray the Asr prayer while the sun is still alive, bright and fresh, that is, before it turns yellow; pray the Maghrib prayer at the time the fasting person breaks his fast, that is, as soon as the sun sets; delay the Isha prayer as long as you do not fear that the people will fall asleep; and pray the dawn (Fajr) prayer in the darkness that lingers after daybreak, and lengthen the recitation in it."
Imam Ishaq bin Rahwayh recorded this hadith in his Musnad, and the narrators in its chain are thiqat (trustworthy).