Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "When you come to your wife and wish to go back again, perform ablution as you do for prayer."
This is how al-Musayyab bin Wadih narrated it, and it is not mahfuz (preserved).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14089