Musannaf Ibn Abi Shayba 28:606

'Abd al-Rahim ibn Sulayman narrated to us, from Hajjaj ibn Artah, from 'Abd al-Rahman ibn Zayd ibn Jud'an, from Yahya ibn Abi Muti', that Abu Bakr al-Siddiq sent an army and said: "Go on a military expedition in the name of Allah. O Allah, make their death martyrdom in Your path. Then, you will come upon a people who are in hermitages (monasteries) for themselves, so leave them to what they have busied themselves with. And you will come upon a people who have shaved the middle of their heads, like a band (of hair). So strike what they have shaved of the middle of their heads."