Riyad as-Salihin 152

While the Prophet (sallallahu alayhi wa sallam) 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 (sallallahu alayhi wa sallam) said, “Tell him to speak, to go into the shade, to sit and to complete his fast”.