Musnad Ahmad 938:12

Jarir narrated to us, from al-A'mash, from Thabit ibn 'Ubayd, who said: Zayd ibn Thabit said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "Do you know Syriac? For letters come to me [in it]." I said: No. He said: "Then learn it." So I learned it in seventeen days.