Musnad Ahmad 130 — Musnad of Umar ibn al-Khattab

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