Abdullah bin Umar (may Allah be pleased with them both) said that the Prophet (peace and blessings be upon him) said: "When the sun begins to rise, delay the prayer until it has fully risen, and when the sun begins to set, delay the prayer until it has fully set."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4380