Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: People used to regard performing Umrah during the months of Hajj as one of the gravest acts of wickedness on earth. They used to call Muharram Safar, and they would say: "When the sores have healed, the traces have faded, and Safar has ended, then Umrah becomes lawful for whoever wishes to perform it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10860