Sahih Ibn Khuzaymah

Sayyiduna Abu Hurairah رضي الله عنه narrates from the Noble Prophet صلى الله عليه وسلم that he صلى الله عليه وسلم said: "Something equal to the back stick of a camel saddle is sufficient as a sutrah, even if it is as thin as a hair." Imam Abu Bakr رحمه الله says that I fear Muhammad bin Qasim made a mistake in narrating this as a marfu' (attributed to the Prophet). Imam Abu Bakr رحمه الله says that from the ahadith of the Noble Prophet صلى الله عليه وسلم, there is strong evidence that he صلى الله عليه وسلم intended the length of the back stick of a camel saddle (for the sutrah), not its width. Among these narrations of the Noble Prophet صلى الله عليه وسلم is that a spear would be planted for him صلى الله عليه وسلم and he صلى الله عليه وسلم would use it as a sutrah while praying, and the width of a spear is not like the width of the back stick of a camel saddle.