Musnad Ahmad 500:1

'Abd al-Razzaq narrated to us, Ibn Jurayj and Muhammad ibn Bakr narrated to us, he said: Ibn Jurayj informed me, he said: Ibn Tawus informed me, from his father, from Abu Isra'il, he said: The Prophet, peace and blessings of Allah be upon him, entered the mosque while Abu Isra'il was praying. It was said to the Prophet, peace and blessings of Allah be upon him: "That is him, O Messenger of Allah; he does not sit, he does not speak to people, he does not seek shade, and he intends to fast." The Prophet, peace and blessings of Allah be upon him, said: "Let him sit, let him speak to the people, let him seek shade, and let him fast."