四分律行事鈔資持記 3.1131

3.1131

《僧祇》，前指七日。次明事訖，初示名。必下，簡緣。故下，引法。天竺夏分多雨，故云雨安居，即召夏初為雨時是也。

The Mahāsāṃghika: the previous points to seven days. Next, it explains the conclusion of the matter; first, it indicates the name. From "Must..." onward, it selects the conditions. From "Therefore..." onward, it cites the rite. In India the summer season has much rain, hence it is called the rain retreat (varṣāvāsa), meaning that the beginning of summer is called the rainy season.