Ibn 'Abbas (May Allah be pleased with them) reported:
While the Prophet (ﷺ) was delivering Khutbah (religious talk), he noticed a man who was standing, so he asked about him and was told that he was Abu Israel who had taken a vow to remain standing and not sit, or go into the shade, or speak while observing fasting. Thereupon Messenger of Allah (ﷺ) said, "Tell him to speak, to go into the shade, to sit and to complete his fast".[Al- Bukhari].
Arabic/English book reference: Book 1, Hadith 152