It is narrated from Hazrat Abu Dharr (may Allah be pleased with him) that a man asked the Holy Prophet (peace and blessings be upon him) about fasting, so the Holy Prophet (peace and blessings be upon him) instructed him to fast on the 13th, 14th, and 15th of the month.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21335
Hadith Gradingحکم دارالسلام:حديث حسن، وهذا إسناد ضعيف