The Prophet (peace and blessings be upon him) used to throw the pebbles at midday when the time for Zhuhr begins. Based on the Prophet’s act, the majority of Muslim scholars state that it is not permissible for pilgrims to throw the pebbles in the morning. However, the Hanafis and other contemporary scholars state that it is permissible to throw the pebbles before the sun reaches its zenith. Muslims can choose the opinion that suits them, keeping in mind that Islam is the religion of easiness.
Concerning this, Salman Ibn Fahd Al-`Udah, a well-known Saudi scholar, states the following:
It is established that the Prophet (peace and blessings be upon him) would wait until the sun had passed its zenith, that is when it had reached midday when the time for Zhuhr begins, during the days of tashreeq (Dhul-Hijjah 11, 12, and 13) and then throw the pebbles. This is reported in Sahih Al-Bukhari on the authority of Ibn `Umar (may Allah be pleased with him and his father) and also on the authority of Jabir (may Allah be pleased with him); it is reported as well in Sahih Muslim.
Based on this, the majority of scholars agreed that it is not permissible for pilgrims to throw the pebbles in the morning. But considering the state of pilgrims on the departure day (that is, the day when pilgrims leave Mina), the Hanafi scholars said that the pilgrims may throw the pebbles on that day before the sun reaches its zenith. So do believe some other scholars like Sheikh ibn Jibrin.
However, some scholars of the predecessors like Ibn `Atta’ and some of the contemporary scholars like Sheikh Ibn Mahmud and Sheikh Abd Ar-Rahman As-Sa`di are of the opinion that it is permissible to throw the pebbles in the morning at all days of tashreeq as there is no religious text that specifies fixed time for throwing the pebbles.
In my point of view, throwing the pebbles in the morning is better than asking someone to throw pebbles on one’s behalf or giving up throwing them on the pretext of over-crowdedness. In addition, throwing in the morning is better, anyway, than risking death underfoot due to the large numbers of pilgrims [and their thoughtless attitude towards one another].