If the adhān is called before its appropriate time, it is considered invalid. This is confirmed through evidence and reasoning.
As for the evidence, it is the saying of the Prophet (صلى الله عليه وسلم): “If the time for ṣalāh arrives, then let one of you call the adhān…”1 He said: “If the time for ṣalāh arrives”, which cannot occur except once the time of the actual ṣalāh has begun. We may also deduce from the wording of this ḥadīth that the adhān may be called only once the time of the ṣalāh arrives and the people intend to perform it imminently.
For this reason, when Bilāl (رضي الله عنه) was travelling with the Prophet (صلى الله عليه وسلم) in extreme heat and, upon the sun descending from its zenith, he stood to call the adhān, the Prophet (صلى الله عليه وسلم) said: “Seek a cooler time.” So, he waited until again he stood to call it and the Prophet (صلى الله عليه وسلم) said: “Seek a cooler time”. They waited until they could see the afternoon shadow cast by the hills. Rather, even later than that, close to the time when the afternoon shadow was nearly equal to the height of the hills themselves. That is, close to the time of ʿAṣr. Then, he (صلى الله عليه وسلم) ordered him to call the adhān.2 This proves that the adhān should be called when the people actually intend to perform the ṣalāh.
We can further build on this concept and say that: if a group of people are travelling or are on an excursion and wish to pray ṣalāh al-ʿIshāʾ, but want to delay its performance until its best time, which is at the end of its appointed period, they should only call the adhān when they intend to perform the ṣalāh itself, not when the time for ʿIshāʾ begins.
As for reasoning: The adhān is meant as a notification of the commencement of the time for a particular ṣālāh. Notification of the beginning of a thing can only occur after its time has commenced. Considering this, if someone calls the adhān before its appropriate time out of ignorance, we would say to him: If the time begins, then repeat the call. This is an occurrence that happens occasionally as some people may be misled by their watches. This person is still rewarded for the adhān he called before the time, as he has engaged in remembrance [which is deserving of reward at any time].
 Authentic: narrated by al-Bukhārī: 11 and Muslim: 292.
 Authentic: narrated by al-Bukhārī: 629 and Muslim: 616.
Source: Al-Sharḥ al-Mumtiʿ 2: 73-74
Translated by: Riyāḍ al-Kanadī