Sahih Bukhari Volume 3, Book 48, Hadith Number 849.

Narated By Said bin Jubair : A Jew from Hira asked me which one of the two periods Musa (i.e. Prophet Moses) completed. I said, “I don’t know, (but wait) till I see the most learned ‘Arab and enquire him about it.” So, I went to Ibn ‘Abbas and asked him. He replied, “Moses completed the longer and better period.” Ibn ‘Abbas added, “No doubt, an apostle of Allah always does what he says.”