Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2527
Hakam b. 'Araj reported: I asked Ibn Abbas (Allah be pleased with them) as he was reclining using his cloak as a pillow near Zamzam about fasting on 'Ashura. The rest of the hadith is the same.