Musannaf Ibn Abi Shayba 3:1442

Abu Bakr narrated to us, he said: Yahya ibn Abi Bukayr narrated to us, he said: Zuhayr ibn Muhammad narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Sa'id ibn al-Musayyib, from Abu Sa'id al-Khudri, that he heard the Prophet, may Allah bless him and grant him peace, saying: "When you stand for prayer, straighten your rows and close the gaps, for I see you from behind my back."