Fiqh-us-Sunnah Volume 5, Hajj and ‘Umrah, Fiqh 5.103.

Section : Its Legality, Throwing Pebbles.

The majority of scholars is of the opinion that throwing the pebbles is necessary (wajib), but it is not obligatory. Anyone failing to throw pebbles is required to offer a sacrifice as reported by Ahmad, Muslim, and Nasa’i from Jabir who said, “I saw the Prophet (peace be upon him) riding his mount and throwing pebbles on the Day of Nahr (10th of Zhul-Hijjah), and saying, ‘Take your rituals from me. I do not know whether I will be able to perform another Hajj after this one’.”

Abdur Rahman Al-Taimi said: “The Prophet (peace be upon him) commanded us in the Farewell Hajj to use small pebbles (the size of a pea).”This was reported by At-Tabarani in his Al-Kabir with a sound chain of authorities.