Musnad Ahmad 27:88

Al-Walid narrated to us, Sa'id ibn 'Abd al-'Aziz narrated to us, from Sulayman ibn Musa, from Nafi', the freed slave of Ibn 'Umar, that Ibn 'Umar heard the sound of a shepherd's reed-pipe. He placed his fingers in his ears and turned his riding beast away from the path. He was saying, "O Nafi', do you hear?" I would say, "Yes." So he would continue until I said, "No." Then he would remove his hands and return his riding beast to the path. 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."