Kharashah bin al-Hurr narrated: I saw Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) striking people's hands over the fasts of Rajab until they put them into the food, and he would say: "Rajab! What is Rajab? Rajab is a month which the people of the Era of Ignorance used to venerate; when Islam came, it was abandoned."
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat. Its chain contains a narrator, al-Hasan bin Jabalah; I have not found anyone who mentions him. Its remaining narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5152