Abu Qatadah al-Adawi (may Allah have mercy on him) narrates from Sayyiduna Umar (may Allah be pleased with him) that he wrote to one of his governors: "Three things are major sins: combining prayers without an excuse, fleeing from the battlefield, and robbery."
Abu Qatadah al-Adawi lived in the time of Umar (may Allah be pleased with him); if he was present when he wrote it, the report is connected (mawsul); otherwise, when it is joined to the first report, it becomes strong. A connected hadith on this has also been narrated from the Prophet (peace and blessings be upon him), but its chain contains someone who is not used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5560