Hi there, preferably boiled water if I’m not mistaken. By giving him/her a few sips of water a couple times a day (not more than 2 ounces per 24 hours) once your baby starts their solid in order to prevent constipation is fine I think.
However, an exclusively breastfed babies at this age do not need additional water. Breastmilk is 88% water and will provide all the fluids that your baby needs.
Do consult your doctor/paediatrician for further advice and guidance :)