It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "When the edge of the sun begins to appear, delay the prayer until it has fully emerged; and when the edge of the sun begins to set, delay the prayer until it has fully set."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13201