It is narrated from Sayyiduna Abu Sa'id (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When a man comes to his wife at night and then intends to come to her again, let him perform ablution."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 984