Sahih Bukhari Volume 008, Book 075, Hadith Number 349.
Narated By 'Ikrima : Ibn 'Abbas 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 Apostle and his companions always avoided it."