[Al-Kafi 4:561, Bihar al-Anwar 41:182, Manaqeb Al Khawarezmi 217]
Ibrahim bin Hayyan narrated from Um Jaafar, Muhammad al-Hanafiyya's wife, from Asma Bint Umais, who said,
'I used to go with the Messenger of Allah (saw) to the battles.
I (Um Jaafar) said, 'What was your responsibility?'
She replied, ''I used to distribute water and take care of the wounded.
In one of the battles, the Prophet (saw) received a revelation, so Ali placed the head of the Prophet on his lap. (due to the process of receiving revelation being physically very taxic for the Prophet. Sometimes during the process, he would sweat profusely).
When the Prophet woke up, the sun had already set.
So the Prophet (saw) asked Ali, 'O Ali! Have you prayed Asr?'
Ali replied ,'No, I did not want to remove your head and place it on the ground.'
So the Prophet (saw) said, 'O Allah! Make the sun return for Ali.' The sun came back again until he had finished praying.''