Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2517
This hadith has been narrated on the authority of Zuhri with the same chain of transmitters that be heard Allah's Apostle (may peace be upon him) as saying on a similar day: I am fasting today, so he who wishes to observe fast should do so; but he did not make mention of the rest of the hadith.