All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The scholars held different opinions regarding the person who fails to throw one pebble (out of the seven). Some said that he is required to slaughter an animal, which is the view held by Malik. Some others said that he is required to donate a mudd (0.75L) of food, which is the opinion of Ash-Shaafi‘i. Abu Haneefah said, "For each pebble that he has forgotten to throw, he should feed a poor person half a sa‘ (a sa‘ is 3L) of wheat. Some other scholars held that nothing is due upon him."
Ahmad said, "If he fails to throw one or two pebbles, there is no problem."
This view was held by Mujahid and Is-haq and chosen by Ibn Hazm. We have previously stated that the opinion of Ash-Shaafi‘i is the preponderant one.
Accordingly, you are required to give a mudd of food in charity at the Sacred Mosque. This is because having doubt during the performance of worship should be taken into consideration. So, you should base your act on certainty, which is the lesser number of thrown pebbles – considering that you failed to throw one pebble.
Allaah Knows best.