It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) entered the mosque on Friday and found Abu Isra'il standing in the sun; he would not go into the shade. The Prophet (peace and blessings be upon him) asked: "What is the matter with him?" The people said: "He has vowed that he will stand, will not go into the shade, and will not break his fast." He (peace and blessings be upon him) said: "Tell him to sit down, go into the shade, and break his fast."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10884