Jarir bin `Abdullah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) asked me on the occasion of the Farewell Pilgrimage to tell the people to keep silent, then he (PBUH) said, "Do not revert to disbelief after me, chopping the heads of one another."[Al-Bukhari and Muslim].
Commenting on this Hadith, Imam An-Nawawi has argued that one is allowed to lend ear to somebody if he has something reasonable to say. This will not be considered as eavesdropping which is unallowable on doctrinal basis. Secondly, silence is a prerequisite of setting ears to a sermon. And a Khatib (one who delivers a sermon) is perfectly justified if he asks his audience to observe silence so as to be able to listen to him in real earnest.