Whoever swears by the Amānah (trust) is not one of us

It is prohibited to swear by other than Allah Almighty, and this includes swearing by the Amānah, as this constitutes an act of minor Shirk (polytheism).
Amānah includes obedience, worship, trust, money, and security.
The oath is not effective or binding unless sworn by Allah Almighty or by one of His names or attributes.
Al-Khattābi said: The undesirability here is likely based on his command that one should swear by Allah and by His attributes, whereas the Amānah is not from His attributes; rather, it is one of His commands and one of the things that He has made obligatory. Therefore, they are prohibited from this because it implies equality between the Amānah and the names and attributes of Allah Almighty.