Abu Mijlaz (may Allah have mercy on him) said that a man asked Sayyiduna Ibn Umar (may Allah be pleased with them both): I have stoned the Jamrah, but I do not remember whether I threw six pebbles or seven. He said: Go to that man, meaning Sayyiduna Ali (may Allah be pleased with him). So he went and asked him, and he said: If this happened to me in my prayer, I would repeat my prayer. He came back and told him (Sayyiduna Ibn Umar (may Allah be pleased with them both)), and he said: Abu al-Hasan has spoken the truth.
The Shaykh (may Allah have mercy on him) said: Perhaps, and Allah knows best, he meant repeating what is in doubt; likewise in stoning, the stoning about which there is doubt is to be repeated.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9668
Ibn Abi Najih (may Allah have mercy on him) said: Tawus (may Allah have mercy on him) was asked about a man who left out one pebble, and he said: He should feed (someone) a morsel. I mentioned this to Mujahid (may Allah have mercy on him), and he said: Abu Abd al-Rahman (Tawus) did not hear the statement of Sayyiduna Sa'd (may Allah be pleased with him). Sayyiduna Sa'd bin Malik (may Allah be pleased with him) said: We returned from the Hajj of the Prophet (peace and blessings be upon him), and some of us would say: I threw six pebbles, and some of us would say: I threw seven pebbles, and none of us found fault with the other for that. [Da'if (weak). Nasa'i: 3077, Ahmad: 1/168; Mujahid (may Allah have mercy on him) did not hear from Sayyiduna Sa'd (may Allah be pleased with him)].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9669