Musnad Abu Ya'la al-Mawsili 111:40

Abdullah ibn Umar narrated to us, Abd al-Rahim ibn Sulayman narrated to us, from Ash'ath ibn Siwar, from Salamah ibn Kuhayl, from Sa'id ibn Jubayr, from Ibn Abbas, who said: A sheep passed in front of the Prophet, may Allah bless him and grant him peace, while he was in prayer, between him and the Qiblah, and he did not interrupt his prayer.