We would like to know what is the ruling of agha Sistani regarding keeping a cat at home as a pet and for namaaz, changing the clothes and then praying. Will the namaaz be valid?

If there is one cat hair or there are two or three hairs on a person's body or clothes, his prayer is in order but if there are more than that, his prayer becomes void, as an obligatory precaution. However, if one becomes aware after prayer, his previous prayer(s) are valid.