The Messenger of Allah ﷺ said to me, "Do you know Syriac well? Letters come to me in it."I said, "No."He said, "Then learn it."So I learned it in seventeen days.
حدثنا جرير عن الأعمش عن ثابت بن عبيد قال قال
لي رسول الله ﷺ تحسن السريانية إنها تأتيني كتب قال قلت لا قال فتعلمها فتعلمتها في سبعة عشر يوما
No references are available.
Its isnad is sahih. Thabit bin Ubayd is the freed slave of Zayd, and he is among the reliable Successors (thiqat tabiin). Tirmidhi narrated the hadith with similar wording in the Book of Seeking Permission, 5/67, no. 2715, and said, "hasan sahih." Bayhaqi also narrated it, 6/211, and Bukhari narrated it with different wording, 13/185 (Fath al-Bari).