Bukhari :: Book 3 :: Volume 31 :: Hadith 137

Narrated Ibn 'Umar:

The Prophet said, "We are an illiterate nation; we neither write, nor know accounts. The month is like this and this, i.e. sometimes of 29 days and sometimes of thirty days."

Source materials are from the University of Southern California MSA site
Hadith eBooks converted from Imaan Star