Musannaf Ibn Abi Shayba 1:47

Waki' narrated to us, he said: Al-A'mash narrated to us, from Salim, from Yazid ibn Bishr, who said: "Indeed, Allah revealed to Musa: 'Perform ablution, for if you do not do so and a calamity befalls you, then do not blame anyone but yourself.'"