Sayyiduna Qatadah (may Allah have mercy on him) says concerning the saying of Allah Almighty "at the declining of the sun" [Surah al-Isra: 78]: When the sun inclines from the middle of the sky, at the time of the Zuhr prayer, until the darkness of the night; and he said: The beginning of the night is the Maghrib prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1680