Mishkat al-Masabih 3147

Ibn ‘Abbas said: One makes atonement for something he has made unlawful for himself.* You have had a good example in God’s Messenger. * i.e. something not in itself unlawful. It is treated like an oath for breaking which atonement must be made. The Prophet’s example refers to his withdrawing from his wives for a period. (Bukhari and Muslim.)