Sayyiduna Samurah (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: "If two partners sell (an item), then the sale conducted first will be valid, and if two guardians marry off (a woman), then the marriage conducted first will be valid."
Hadith Referenceالمنتقى ابن الجارود / كتاب البيوع والتجارات / 623