We explore the origin of business cycle synchronization through a unique administrative dataset on plant-level sales with regional and time-level heterogeneity in India. Regional sales exhibit a high level of synchronization that can be traced to comovement in a small number of large plants located in different regions, indicating a granular nature of regional synchronization. Quantitatively, the top 10 plants from each region explain almost two-thirds of the total variation in regional GDP correlation. We test three channels for synchronization, viz. through aggregate regional trade, large granular plants, or granular multi-plant firms located in different regions. We find that inter-region sales of multi-plant firms is the most dominant factor behind regional GDP synchronization and subsumes both aggregate trade and plant-level effects.