In The Name of Allaah, The Most Merciful, The Betower of Mercy.
Shaikh Uthaymeen [rahimahullaah] said: This precaution is not legislated and indeed the Prophet [sallal-laahu-alayhi-wasallam] said: “The over-stringent ones are destroyed”. [Ref 1]
It [i.e. this type of precaution] is incorrect because if they [i.e. the people] take this precaution in relation to the fasting, they will commit a mistake or a misdeed. That is because many people stand up and [start] praying Fajr when they hear the Mu’addhin. Therefore, the one who hears the A’dhaan of the Mu’addhin before Fajr and starts praying, he might have prayed before the right time and prayer before its right time is incorrect. So in this regard, the Musalloon [performers of the prayer] have been wronged.
Likewise, the Saa’imoon [i.e. those who intend to perform the fast on that day] are wronged because he [Mu’addhin] prevents them from eating and drinking despite the fact that it is permissible [i.e. eating and drinking until the correct time of Fajr]. He [the Mu’addhin] commits an offence against the Saa’imoon because he prevents them from that which Allaah has made permissible for them; and he commits an offence against the performers of the prayer when they pray before the right time of Fajr and this is a nullifier of the prayer. Therefore, it is obligated on the Mu’addhin to fear Allaah [The Mighty and Majestic] and that he follows- whilst aspiring to establish what is correct- that which the Book and Sunnah guides to. [Source: An Excerpt from ‘Majmoo Fataawaa’ of the Shaikh (rahimahullaah) 19/295]
Ref 1: The Messenger (sallal-laahu-alayhi-wasallam) said, ‘’The over-stringent ones are destroyed’’: Imaam Nawawi (rahimahullaah) said: ”The over-stringent ones are those who look too deeply (into affairs) and are excessive- those who exceed the Hudood [prescribed legislated sharia boundaries] in their statements and actions.” [Source: Saheeh Muslim Hadeeth Number 2670 & Saheeh Muslim Bi-Sharh An-Nawawi Vol 16, Page 180 & page 220. Publisher Dar Al-Kotob Al-Ilmiyyah 1st ed 1421AH (Year 200)]