Eigenvalues of sum of commuting matrices [closed]

With reference to the following thread :

Eigenvalues of Matrix Sums

Answer by Jonas Meyer is as follows :

If 2 positive matrices commute, than each eigenvalue of the sum is a sum of eigenvalues of the summands. This would be true more generally for commuting normal matrices. For arbitrary positive matrices, the largest eigenvalue of the sum will be less than or equal to the sum of the largest eigenvalues of the summands.

Can you suggest a reference or source for this.

I am trying to prove that the eigen values of a matrix are all negative and if the above statement holds then it will be of great use. So, please point the references for the same.