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
(a) It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "When any of you intends to return (to his wife) again, let him perform ablution, for it gives more vigour for returning."
(b) It is narrated from Umar bin al-Khattab (may Allah be pleased with him) that he ordered ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 985