Musnad Ahmad

It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that such people have borne witness to me, whose word is trustworthy, among them Sayyiduna Umar (may Allah be pleased with him) is also included, who in my view is the most trustworthy of them all, that the Prophet (peace and blessings be upon him) used to say: "There is no prayer after two prayers: after the Fajr prayer until the sun rises, and after the Asr prayer until the sun sets, no voluntary prayer should be performed."