It is narrated from Imam Musa ibn Jafar Al-Kadhim عليه السلام who said: "Whoever fasts the first day of the ten days of Dhu al-Hijjah, Allah سبحانه وتعالى will record for him the reward of fasting eighty months. And if he fasts all nine days, Allah سبحانه وتعالى, the Almighty, will record for him the reward of fasting a lifetime."