Abu Suhayl bin Malik narrates from his father that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) wrote to Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him): Lead Zuhr when the sun has declined; Asr while the sun is white and clear, before any yellowness enters it; Maghrib when the sun has set; Isha as long as you have not slept; and lead Subh while the stars are still visible, and recite in it two long surahs of the Mufassal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1729