Mixing formula and breast milk...help!

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My husband in an effort to be helpful premixed a bottle of breast milk and formula (were currently doing both). We’re using the enfamil gentlease and it’s liquid ready to use, not powder. My question is is it safe to premix it like this and store it in the fridge for a few hours? From what I could find online I believe it says that if it’s refrigerated and mixed it’s good for 24 hours. Any advice is appreciated, thank you!