What is the ruling of Islamic law regarding a mother's refusal to breastfeed her three-month-old child, and her starting to give him formula milk from the pharmacy instead, due to her preoccupation with work and her intentional cessation of milk production?

The basic principle is that a woman should breastfeed her child for two full years. However, weaning before that is permissible if the child is not harmed. If the child is content with formula milk or other alternatives and is not harmed by its use, then there is no legal impediment.