Musnad Ahmad 1026:2

'Abd al-Rahman ibn Mahdi narrated to us, from Malik, from Sumayy, from Abu Bakr ibn 'Abd al-Rahman, from one of the companions of the Prophet (may Allah bless him and grant him peace): "The Prophet (may Allah bless him and grant him peace) was seen at al-'Arj pouring water over his head while he was fasting, due to the heat or thirst."