"that in which you are believers." Al-Hasan's explanation:

Three individuals from the Companions of the Prophet—one of them vowed to himself never to approach women, another vowed to himself never to break his fast during the day, and another vowed to himself never to sleep at night. 'Uthman ibn Maz'un was among those who vowed to himself never to approach women. His wife would visit the wives of the Prophet while looking well-groomed and smelling pleasant. When 'Uthman made his vow, she came to them not in that state, so they inquired about it. She said, "A woman only adorns herself for her husband; and such-and-such, and such-and-such have made such-and-such vows upon themselves." When the Messenger of Allah arrived, they mentioned that to him. He became angry and sent for them, then said, "Have I not been told about you regarding such-and-such?" They replied, "Yes." He said, "But I fast and break my fast, I stand in prayer and I sleep, and I approach women and I refrain. So whoever turns away from my Sunnah is not of me." The people then sought forgiveness for that and reverted to their original practice.

Surat al-Ma'idah from verse (89).