It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "When the edge of the sun begins to rise, do not perform prayer until it becomes fully apparent, and likewise, when the edge of the sun begins to set, do not perform prayer until it has completely set."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4694
Hadith Gradingحکم دارالسلام:إسناده صحيح، خ : 582، م : 828