Maʿānī al-ʾAkhbār #575Shawwal, Dhu’l-Qa’da and Dhu’l-Hijjah. In another narration: And one singular month for Umrah, Rajab.
Al-Kāfi #2021“Abu ‘Abd Allah (a.s.), has said, ‘If one says Talbiyah for al-‘Umrah in the month of Rajab and comes out of the state of Ihram in another...
Al-Kāfi #2024“One may perform al-‘Umrah during any month of the year. The best al-‘Umrah is the al-'Umrah of (the month of) Rajab.”