Sayyiduna Qurrah bin Iyas (may Allah be pleased with him) narrated: The Prophet (peace and blessings be upon him) said: "Fasting three days of every month is like fasting perpetually, and it is also not fasting perpetually (i.e. the reward will be that of fasting perpetually, while in practice one will not have fasted perpetually)."
Imam Ahmad, Imam al-Bazzar and Imam al-Tabarani in al-Mu'jam al-Kabir recorded it. The narrators of Imam Ahmad are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5185
Hadith Gradingمحدثین:إسنادہ صحیح
Hadith Takhrij«اخرجه الطبراني فى المعجم الكبير (26/9) اخرجه احمد فى المسند (435/3 و 436 و 19/4 و 34/5 و 35) اخرجه البزار فى المسند (1059) ، أخرجه مسند أحمد برقم: 15816، وأخرجه سنن الدارمي برقم: 1747»