Mishkat al-Masabih 7 Fasting كتاب الصوم 55

وَعَنْ بَعْضِ أَصْحَابِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: لَقَدْ رَأَيْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِالْعَرْجِ يَصُبُّ عَلَى رَأْسِهِ الْمَاءَ وَهُوَ صَائِمٌ مِنَ الْعَطَشِ أَوْ مِنَ الْحَرِّ. رَوَاهُ مَالك

One of the Prophet’s companions said he had seen the Prophet in al-‘Arj* pouring water over his head while he was fasting; either because of thirst or because of the heat. *A wadi in the neighbourhood of at-Ta’if. Malik and Abu Dawud transmitted it.