First, a valid source using empirical evidence needs to assess the claim.

The rules consider only physical media as a valid source and they must be followed very strictly.

The group recognised only Shari'a as a valid source of law.

His works (more than 300 volumes) are used as valid sources in Jurisprudence.

Having read the blog there is very little hard evidence or valid sources to support his claims.

Peer-reviewed papers are the only scientifically valid sources of information out there.

One of the most important factors to consider when dealing with color calibration is having a valid source.

They held perception to be the only valid source of knowledge.

Authentication - to verify that the message is from a valid source.

There are no 100% valid or trustworthy source that can be found on how powerful a shot from this weapon is.