(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