Musnad Ahmad 355 — 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) forbade praying at two times: one is that after the Asr prayer until sunset, no voluntary prayer should be performed, and the other is that after the Fajr prayer until sunrise, no prayer should be performed.