Hekayat 3 from Chapter 4 of Golestan of Saadi Shirazi

A wise young man was well-versed in various disciplines of virtue, yet had an introverted nature. Whenever he sat in gatherings of scholars, he kept his speech to himself. One day, his father said: "My son, why don't you also speak about what you know?" The son replied: "I fear they will ask about what I do not know, and I will be embarrassed."

Have you not heard about the Sufi who was hammering some nails under his sandals?
A soldier grabbed his sleeve and said: 'Come and shoe my horse.'

جوانی خردمند از فنون فضایل حظی وافر داشت و طبعی نافر. چندان که در محافل دانشمندان نشستی زبان سخن ببستی. باری پدرش گفت: ای پسر! تو نیز آنچه دانی بگوی. گفت: ترسم که بپرسند از آنچه ندانم و شرمساری برم.

نشنیدی که صوفی‌یی می‌کوفت

زیر نعلین خویش میخی چند

آستینش گرفت سرهنگی

که بیا نعل بر ستورم بند