Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) stood for the people in the year of the Farewell Pilgrimage, and the people were asking him about rulings. A man came and said: O Messenger of Allah! I did not realise, and I sacrificed before stoning. He (peace and blessings be upon him) said: "Stone, and there is no harm." Another said: I did not know, and I shaved my head before sacrificing. He (peace and blessings be upon him) said: "Slaughter, and there is no harm." He said: That day, whenever the Prophet (peace and blessings be upon him) was asked about doing any act earlier or later, he (peace and blessings be upon him) gave this same answer: "Do it, and there is no harm."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9620