Musnad Ahmad 31:239

Muḥāḍir ibn al-Mura‘ narrated to us, ‘Āṣim ibn Sulaymān narrated to us, from Abū al-Mutawakkil, from Abū Sa‘īd al-Khudrī, from the Messenger of Allah (peace and blessings of Allah be upon him), who said: "If one of you approaches his wife, then wishes to return [to her], let him perform ablution like his ablution for prayer."