Sahl ibn Saʿd [may Allāh be pleased with him] reported,
“If he (the Prophet) was fasting he would order a man to go upon high ground (to look out for the sunset), and when he says “the sun has set”, he would break his fast”
In this hadīth is an indication of the importance and concern of the Prophet [ṣallAllāhu ʿalayhi wa sallam] in hastening to break the fast after confirming and making sure that the sun had set. He would order someone to ascend a high ground, and inform him of the setting of the sun so he may break his fast. He did not do this except to actualise his statement:
“The people will remain upon good as long as they hasten to break the fast”
(Agreed Upon, Irwāʾ al-Ghalīl: 917)
Today, what is truly regrettable is that we see the people opposing this Sunnah. For many of them see with their eyes that the sun has set, but they still do not break the fast until they hear the adhān of the country.
They are ignorant of two issues:
First: the adhān is not given upon seeing the sun set, rather according to astronomical timings.
Second: the setting of the sun in one country may differ from one place to another due to the mountains and valleys. So we have seen people not breaking the fast while they themselves have seen the sun set! And others break the fast while the sun is clearly visible and has not set, all because they have heard the adhān! All help is sought from Allāh!
Reference: Silsilah al-Aḥādīth al-Ṣaḥīḥah, volume 5, page 118, no. 2081