Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Throwing pebbles (Rami) before the sun passes the meridian is not correct according to al-Malikiyah, al-Shafi’eyah and al-Hanabilah. This is also the known narration and practice in the Hanafi school of thought. However, there is another opinion in Hanafi school of thought that permits throwing pebbles before the sun passes its meridian. But Ahnaf permitted that for the one who decided to leave Mina on the tenth Zil-Hijjah or thirteenth Zil-Hijjah (the day, the pilgrim wants to finish his stay in Mecca) to throw the pebbles before noon. It is also reported from Imam Ahmad in a narration from him.
The evidence of the majority of Muslim scholars is that the Prophet (Sallallahu Alaihi wa Sallam) did not perform Rami until after noon.
As a general rule the matter of worship is not subject to Ijitihad and should be performed as reported.
Those who permitted throwing pebbles before noon compare it with the Rami of the tenth Zil-Hijjah which is done before noon. However, the majority of Muslim scholars are of the opinion Rami is not allowed before Zawal (noon) but the Rami of the previous day is allowed before Zawal (noon). However, the matter is flexible.
Know that when we say that the time of Rami starts after the sun passes the meridian, it means that a pilgrim may throw pebbles from Zawal till sunset and from Maghrib till Fajr but he may even throw in the remaining days of Tashreeq. If one observes the matter he may achieve many things.
First, follow the Sunnah in throwing pebbles after Zawal.
Second, be on the safe side since the majority of Muslim scholars are of the opinion that Rami before noon is not correct.
Third, avoid hardship since the time extends to a whole night and all the Days of Tashreeq.
Allah knows best.