Musnad Ahmad 27:516

Al-Walid ibn Muslim narrated to us, Sa’id ibn ‘Abd al-‘Aziz and Mukhallad ibn Yazid narrated to us, Sa’id informed us—the meaning being the same—from Sulayman ibn Musa, from Nafi’, the freed slave of Ibn ‘Umar: Ibn ‘Umar heard the sound of a shepherd’s reed pipe, so he placed his two fingers in his ears and turned his riding beast away from the path, while he was saying: "O Nafi’, do you hear?" I would say: "Yes." He would continue [walking away] until I said: "No." He said: So he placed his hands [down] and returned the riding beast to the path, and he said: "I saw the Messenger of Allah, may Allah bless him and grant him peace, and he heard the sound of a shepherd’s reed pipe, and he did the same as this."