The Prophet ﷺ said, "Whoever combines two prayers without a valid excuse has entered through one of the gates of the major sins."
حدثنا أبو سلمة يحيى بن خلف البصري حدثنا المعتمر بن سليمان عن أبيه عن حنش عن عكرمة
عن النبي ﷺ قال من جمع بين الصلاتين من غير عذر فقد أتى بابا من أبواب الكبائر
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Its chain of transmission is very daif. Bayhaqi related it at 3/169 from the hadith of Mutamir with the same chain, and Hanash is discarded as a narrator. Bayhaqi also related it from Umar bin Khattab with a similar meaning as a stopped narration, and it is strong according to him.