Stoning the Jamaraat means throwing a certain number of pebbles in the specific places for stoning in Mina (the Jamaraat). It is one of the great rituals of Hajj which the pilgrims do during a certain number of days in Mina. The Jamrah is not the pillar that is found in the middle of the basin surrounding it (the marma), rather the Jamrah is the basin surrounding this pillar. If a person’s pebbles fall inside the basin that is allocated for stoning, then his stoning is valid and is acceptable, according to scholarly consensus. 

Imam al-Shaafa’i (may Allah have mercy on him) said: 

The minimum that is required in stoning is that he should throw so that his pebbles land in the place for pebbles. If he throws a pebbles and cannot see it and he does not know where it fell, he should repeat it, and it does not count until he knows that it has fallen in the place of pebbles. End quote.

Al-Umm (5/235). 

Ibn Qudaamah (may Allah have mercy on him) said: 

This stoning does not count unless the pebble falls in the basin. If it falls outside of it, it does not count according to the view of all scholars, because he is commanded to throw and he has not thrown. 

If he tosses it, it is valid, because that is called throwing. 

This is the view of ashaab al-ra’y, but Ibn al-Qaasim said that it is not valid. 

If he throws a pebble and it is caught by a bird before it lands, that is not valid, because it has not fallen in the basin. 

If it lands on solid ground that is not inside the basin, then it rolls into the basin or onto a person’s clothes then it bounces and falls into the basin, that is valid because it is his action that led to that. 

If he throws a pebble but he is unsure as to whether it landed in the basin or not, that is not valid, because the basic principle is that the throwing still remains to be done, and is not altered in the case of uncertainty. If it appears that it has fallen into the basin, then that is valid, because what is most apparent counts as evidence. 

Al-Mughni 3/219, 220. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him) said: 

It is not essential that the pebble should remain in the basin, but it is essential that it should fall into it. If the pebble landed in the basin and then rolled out of it, that is acceptable according to the apparent meaning of the words of the scholars. Among those who stated that clearly was al-Nawawi in al-Majmoo’. It is not prescribed to stone the pillar, rather the Sunnah is to throw into the basin. End quote. 

Majmoo’ Fataawa Ibn Baaz, 16/144-145 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

One of the conditions of stoning is that the pebble should fall inside the basin. If the pebble falls in the basin, then you have discharged this duty, whether it remains in the basin or rolls out of it. Another mistake made in stoning is when some people think that it is essential that the stone hits the pillar. This is a mistaken notion. In order for the stoning to be valid it is not essential for the pebble to hit the pillar. It is only there to serve as the sign of the basin in which the pebbles fall. If the pebble falls in the basin, then it is valid whether it hits the pillar or not. End all. 

Fiqh al-‘Ibaadaat, p. 383, question no. 279 

He also said (may Allah have mercy on him): 

The aim is that the pebble should fall in the basin, whether it strikes the pillar or not. End quote. 

Al-Sharh al-Mumti’, 7/321 

Shaykh Saalih al-Fawzaan (may Allah preserve him) said: 

It is essential that every pebble should fall inside the basin, whether it stays inside or it falls out of it after that. If it does not fall into the basin then it is not valid. End quote. 

Al-Mulakhkhas al-Fiqhi, 1/446 

See also the answer to question number 34420 

And Allah knows best.