It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that he said: "The Prophet (peace and blessings be upon him) would fast (voluntary fasts) until we would say (among ourselves) that he (peace and blessings be upon him) would not stop fasting now, and he would (at times) stop fasting until we would say that he (peace and blessings be upon him) would not fast now; and he (peace and blessings be upon him) fasted most often in the month of Sha'ban."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from Umar except Ali."
Hadith Referenceالمعجم الأوسط / باب الألف / 1773
Hadith Gradingمحدثین:إسناده ضعيف جدًّا، فيه عمر بن صهبان متروك
Hadith Takhrij«قال الهيثمي: فيه عمر بن صهبان وهو متروك «مجمع الزوائد ومنبع الفوائد، ج 3، ص 192»» «أخرجه الطبراني فى «الكبير» برقم: 5805»