Imam Sadiq (peace be upon him): Mufaddal said: I asked my master Imam Sadiq (peace be upon him): "Does the mission of the awaited Mahdi (may Allah hasten his noble reappearance) have a set time that the people must know?" He said: "Far be it from Allah to set the time of his appearance in a manner that our Shia would know it." I said: "Why, my master?" He said: "Because the time of his appearance is the very Hour of which Allah says: 'They ask you about the Hour, when will it come? Say: Knowledge of it lies only with my Lord. None but He shall reveal it at its time. It weighs heavily on the heavens and the earth. It will not come upon you but suddenly. They ask you as though you knew of it well. Say: Knowledge of it lies only with Allah, but most people do not know' (al-A'raf/187). And this is also the Hour of which Allah said: يَسْئَلُونَکَ عَنِ السَّاعَةِ أَیَّانَ مُرْساها [They ask you about the Hour, when will it come], and He also said: 'Knowledge of that time is only with Allah' (Luqman/34). And in another verse He said: 'Do they wait for anything but that the Hour should come upon them suddenly? Indeed its signs have already appeared. But when it comes upon them, how will their remembrance benefit them?' (Muhammad/18). And He also said: 'The Hour has drawn near and the moon has split' (al-Qamar/1). And He also said: 'It is Allah who has sent down this Book with truth and the Balance. And what will make you know? Perhaps the Hour is near. Those who do not believe in it seek to hasten it, and those who believe are fearful of it and know that it is the truth. Beware! Indeed those who dispute about the Hour are in far-off error' (al-Shura/17-18)."