Allah, exalted be His praise, says: "Establish the prayer from the declining of the sun." [Surah al-Isra: 78]
Imam al-Shafi'i (may Allah be pleased with him) said: "The duluk of the sun means its zawal (declining)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / Q1704