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)."
It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that "the declining of the sun" (duluk al-shams) [Surah al-Isra: 78] means its inclining after midday.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1704