According to Sayyadi Sahl Ibn Abdullah al-Tustari, may Allah be Merciful to him, said:--
"It was narrated on the authority of Ibn Mas'ud, may Allah be pleased with him,
that he [Ibn Mas'ud] said: 'The one who knows the Qur'an by heart should be
known for his vigil at night while people are asleep, for his fasting during the day
while people are eating, for his sorrow while people are rejoicing, for his weeping
when people are laughing, and for his silence when people are talking."
Thus who knows the Qur'an by heart should be tearful, sorrowful, wise and learned, not hard-hearted and deceptive, that is to say, not dishonest.'
"TAFSIR AL-TUSTARI", by
Sahl Abdullah al-Tustari.