Abdullah bin Abbas reported,

They used to consider performing Amra during the months of Hajj to be among the gravest sins on earth, and they would treat Muharram as Safar, saying, "Once the camels' backs have healed, the tracks have faded, and Safar has ended, Amra 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 commanded them to convert it into Amra, and this seemed enormous to them.They asked, "Messenger of Allah, what kind of release is this?"He replied, "A complete release."

Azami 5378

Sahih

عن ابن عباس قال

كانوا يرون أن العمرة في أشهر الحج من أفجر الفجور في الأرض، ويجعلون المحرم صفرا، ويقولون: إذا برا الدبر وعفا الأثر وانسلخ صفر حلت العمرة لمن 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.

الأعظمي 5378صحيح
Contents
Reference
Commentary
Ayat
Collection
Jami al-Kamil
Book
Hajj
Chapter
Umrah
Section
Umrah during the months of Hajj
Numbering
Jami al-Kamil 5378 (2nd edition)
Azami 5378