Musnad Ahmad 937:214

Yahya ibn Ghaylan narrated to us, Rushdin—meaning Ibn Sa'd—narrated to us, 'Amr ibn al-Harith narrated to me, he said: And Rushdin narrated to me, from Salim ibn Ghaylan al-Tujibi, he narrated to him that Sulayman ibn Abi 'Uthman narrated to him, from Hatim ibn Abi 'Adi or 'Adi ibn Hatim al-Himsi, from Abu Dharr, he said: I said to the Messenger of Allah, peace and blessings of Allah be upon him: "I want to spend the night with you tonight, so that I may pray with your prayer." He said: "You cannot [endure] my prayer." Then the Messenger of Allah, peace and blessings of Allah be upon him, stood to perform ghusl (ritual bath), and he was screened by a garment, and I was turned away from him, so he performed ghusl. Then I did the same as that. Then he stood to pray, and I stood with him until I began to strike my head against the walls due to the length of his prayer. Then Bilal gave the adhan (call) for prayer, and he (the Prophet) said: "Have you done it?" He (Bilal) said: "Yes." He said: "O Bilal, you are giving the adhan when the dawn is shining in the sky, and that is not the dawn. The dawn is rather like this, horizontally." Then he called for suhur (pre-dawn meal) and ate suhur.