They used to consider performing Umra during the months of Hajj to be one of the gravest sins on earth, and they would treat Muharram as Safar, saying, "Once the camels' sores have healed, the tracks have faded, and Safar has ended, Umra becomes lawful for whoever wishes to perform it."The Prophet ﷺ and his companions arrived on the morning of the fourth, having entered the state of ihram for Hajj. He instructed them to convert it into Umra. This struck them as a serious matter, so they asked, "Messenger of Allah, what kind of release is permitted?"He replied, "A complete release."
كانوا يرون أن العمرة في أشهر الحج من أفجر الفجور في الأرض، ويجعلون المحرم صفرا، ويقولون: إذا برأ الدبر وعفا الأثر وانسلخ صفر حلت العمرة للمن اعتمر قدم النبي ﷺ وأصحابه صبيحة رابعة مهلين بالحج فأمرهم أن يجعلوها عمرة فتعاظم ذلك عندهم فقالوا: يا رسول الله، أي الحل؟ قال: حل كله.
Bukhari narrated it in Hajj (1564), and Muslim in Hajj (1240:198), both through Wahib, who said, Abdullah bin Tawus told us, from his father, from Ibn Abbas, and he mentioned it. Their wording is the same.In another version with Muslim (199), through Abu Aliyah Bara, from Ibn Abbas, with the same account, it states, "So he arrived when four days of Dhul-Hijjah had passed."