From Sahih Bukhari (Hadith 2684):
Narrated Sa`id 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 inquire 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
Reference: Bukhari 2684