The aim of the study reported in this article was to reveal how instruction, enriched with reasoning about students' thought processes and strategies, regulates prospective teacher's noticing skills in the context of whole number multiplication. In addition, the study examined the evidence of the prospective teacher's (Matt) knowledge that supported the interplay between knowledge and noticing that emerged during the learning process. To this end, the prospective teacher participated in the instruction enriched with reasoning about students' thought processes and strategies. The prospective teacher's noticing skills related to correct and incorrect student-invented strategies were examined before and after his engagement with the instruction provided. The data gathered through the prospective teacher's written responses in the pre- and post-tests and the follow up interviews were analysed with respect to the dimensions of the Professional Noticing of Children's Mathematical Thinking framework. Moreover, to explore the evidence of knowledge that underpins the teacher's noticing skills, data were analysed through the Mathematics Teacher's Specialised Knowledge model. The findings indicated positive change in the prospective teacher's noticing skills for both correct and incorrect student-invented strategies after his involvement in the intervention.