Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) narrated that he was leading the people in prayer; they saw something and laughed. When Sayyiduna Abu Musa (may Allah be pleased with him) finished the prayer, he said: Whoever laughed in the prayer should repeat the prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 676