(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