Musnad Ahmad 1153:1887

Abu Kamil narrated to us, Ibrahim narrated to us, Ibn Shihab narrated to us, from Urwah, who said: I said to Aisha, "Have you seen the saying of Allah, the Almighty and Majestic: 'Indeed, as-Safa and al-Marwah are among the symbols of Allah. So whoever makes Hajj to the House or performs 'Umrah - there is no blame upon him for walking between them' [Al-Baqarah: 158]. By Allah, there is no blame upon anyone if he does not walk between them." She said: "How evil is what you have said, O son of my sister! If it were as you interpreted it, it would have been: 'there is no blame upon him if he does not walk between them.' It was only revealed because this tribe of the Ansar, before they embraced Islam, used to enter into the state of Ihram for Manat, the idol which they used to worship at al-Mushallal. And whoever entered into Ihram for it would feel restricted from walking between as-Safa and al-Marwah. So they asked the Messenger of Allah (ﷺ) about that, and Allah, the Almighty and Majestic, revealed: 'Indeed, as-Safa and al-Marwah are among the symbols of Allah. So whoever makes Hajj to the House or performs 'Umrah - there is no blame upon him for walking between them.'" She said: "Then the Messenger of Allah (ﷺ) established the walking between them as a Sunnah, so it is not appropriate for anyone to abandon walking between them."