Imam Abu Bakr (may Allah have mercy on him) says that in the narration of Sayyiduna Ibn Abbas (may Allah be pleased with them both), the Prophet Muhammad (peace and blessings be upon him) saw Abu Isra’il standing in the sun and asked:“What has happened to him that he is standing in the sun?”The companions replied that he had made a vow to fast, not to sit, and not to seek shade. The Prophet (peace and blessings be upon him) said:“Tell him to sit down, stay in the shade, and complete his fast.”Thus, the Messenger of Allah (peace and blessings be upon him) ordered him to fulfill the vow of fasting because fasting is obedience and servitude to Allah, and forbade him from standing in the sun because standing in the sun is not an act of obedience and servitude to Allah. Although standing in the sun is not a sin, if it causes physical harm, then it becomes sinful. I have explained this matter in detail in the Book of Vows.