In this paper, we propose an integer programming model for obtaining lower bounds for the curriculum-based course timetabling problem, in which weekly assignments for courses into rooms and periods are considered. The model is a pattern formulation where a pattern is an assignment of a course into a set of periods on one day. Different preprocessing techniques are implemented to reduce the number of variables and valid inequalities are derived and added to the model. The proposed model is tested on 21 real-world data instances. On 17 of these instances the best know solutions have been proven optimal and for the remaining four our model improves the lower bounds for three of them.