Proofs from the Glorious Qur'an
"Invite (mankind, O Muhammad peace be upon him) to the Way of your Lord (i.e. Islâm) with wisdom and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His Path, and He is the Best Knower of those who are guided." [Surat An-Nahl: 125].
"He grants Hikmah to whom He pleases, and he, to whom Hikmah is granted, is indeed granted abundant good. But none remember (will receive admonition) except men of understanding." [Surat Al Baqarah: 269].
Proofs from the Hadiths
`Ikrimah reported that Ibn `Abbas (may Allah be pleased with him) said: "Preach to the people once a week, and if you won't, then preach them twice, but if you want to preach more, then let it be three times (a week only), and do not make the people fed-up with this Qur'an. If you come to some people who are engaged in a talk, don't start interrupting their talk by preaching, lest you should cause them to be bored. You should rather keep quiet, and if they ask you, then preach to them at the time when they are eager to hear what you say. And avoid the use of rhymed prose in invocation for I noticed that Allah's Messenger (peace be upon him) and his companions always avoided it." [Reported by Al Bukhari, No. 6337].
Ibn Mas`ud (may Allah be pleased with him) said: "If you speak to some people a speech which  which their minds cannot grasp, it will be a sedition to some of them." [Reported by Muslim in the introduction of his Sahih].
 The Divine Revelation and the Qur'ân.
 The previous Hadith in choosing time, which indicates choosing the information that suit the readiness of students, and both are part from the great principles of education.
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