What could be causing a leak on the supply side of a washing machine hose
There should be a rubber "rubber" washer in the plastic union that screws onto the tap.

(the washer is usually some sort of flexible plastic, not actual rubber)

It's called a hose union washer and should be available at most places that sell hardware or plumbing supplies.

I have in the past used slices of actual rubber hose to replace lost washers.

If there is a piece of fibre in the joint that could cause a slow leak. so when you undo the union to check for the washer, if you do find a washer make sure that it and the surface below it are clean and undamaged, and that the mating face of the tap is also clean.