Sayyiduna Amir bin Rabi'ah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "One Umrah to the next Umrah is an expiation for the sins and faults committed in between, and the reward of an accepted (mabrur) Hajj is nothing but Paradise."
Imam Ahmad recorded this narration; its chain contains a narrator, Asim bin Ubaydullah, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5660