Confusion regarding properties of inner product

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I just had some confusion about linearity of inner product.

In my course notes, it's said that inner product is linear in first slot i.e.

$\langle u+w,v\rangle = \langle u,v\rangle + \langle w,v\rangle$ and $\langle au,v\rangle = a\langle u,v\rangle$

But when I searched for logical explanations, I stumbled upon a video which said that inner product is linear in second argument i.e.

$\langle u,v+w\rangle = \langle u,v\rangle + \langle u,w\rangle$ and $\langle u,\lambda v\rangle = \lambda \langle u,v\rangle$

But neither of the sources said about the other slot/argument.

So, in which slot/argument is inner product actually linear?