(a) Abu al-Aliyah narrates from Sayyiduna Umar (may Allah be pleased with him) that combining (prayers) without an excuse is a major sin.
(b) Imam al-Shafi'i (may Allah have mercy on him) says that the excuse is travel and rain, but this is not established from Sayyiduna Umar (may Allah be pleased with him); it is mursal (its chain is broken).
It is as al-Shafi'i said: the well-known chain of this report is the one we have mentioned, and it is mursal; Abu al-Aliyah did not hear from Umar (may Allah be pleased with him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5559
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
(a) Ikrimah narrates from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Combining prayers without an excuse is a major sin."
(b) In the narration of Ya'qub: "Whoever combines two prayers without an excuse has committed a major sin."
Only Husayn bin Qays Abu Ali al-Rahabi, known as Hanash, narrated it, and he is da'if (weak) according to the scholars of transmission; his report is not used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5561