Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) passed by a boy who was reading: "The Prophet is closer to the believers than their own selves, and his wives are their mothers" [Surah al-Ahzab: 6] "and he is a father to them." He said: "O boy! Erase it." He said: "This is the mushaf of Ubayy (Sayyiduna Ubayy bin Ka'b, may Allah be pleased with him)." So he went to Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) and asked him, and he said: "That is because the Qur'an kept me occupied (I was reciting the Qur'an), while the clamour of the markets kept you busy (that is, buying and selling)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13419