Mishkat al-Masabih 3430
Ibn ‘Abbas said:
While the Prophet was preaching a man was standing, so he asked about him and was told that he was Abu Isra’il who had taken a vow to stand and not to sit, or go into the shade, or speak, but to fast. Thereupon the Prophet said, “Command him to speak, go into the shade, sit and complete his fast.” 

Bukhari transmitted it.
وَعَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: بَيْنَا النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَخْطُبُ إِذا هُوَ بِرَجُل قَائِم فَسَأَلَهُ عَنْهُ فَقَالُوا: أَبُو إِسْرَائِيلَ نَذَرَ أَنْ يَقُومَ وَلَا يَقْعُدَ وَلَا يَسْتَظِلَّ وَلَا يَتَكَلَّمَ وَيَصُومَ فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مُرُوهُ فَلْيَتَكَلَّمْ وَلْيَسْتَظِلَّ وَلْيَقْعُدْ وَلْيُتِمَّ صَوْمَهُ» . رَوَاهُ الْبُخَارِيُّ