Musannaf Ibn Abi Shayba 33:209

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Sulayman ibn Maysarah, from Tariq ibn Shihab, from Rafi’ ibn Abi Rafi’, who said: I accompanied Abu Bakr, and he had a Fadaki cloak which he would fasten upon himself when he rode, and I and he would wear it when we dismounted. And it is the cloak which Hawazin taunted him with, for they said: "When [he uses] the pin, we shall pledge allegiance after the Messenger of Allah, may Allah bless him and grant him peace."