四分律刪繁補闕行事鈔 2.309

2.309

覆頭者，若以樹葉、碎段物、若衣覆也。不犯者：有病、患寒、頭上生創、命梵難，覆頭走者。

Covering the head means covering it with tree leaves, fragmented pieces of cloth, or a robe. One does not commit an offense if: being ill, suffering from cold, having sores on the head, or facing danger to life or pure conduct and running with the head covered.