Fiqh-us-Sunnah Volume 002, Supererogatory Prayer, Fiqh 2.144.

Section : Offering the salah in a crowded area.

Ahmad and al-Baihaqi relate from Sayyar that 'Umar was giving an address and said: "The Messenger of Allah built this mosque and we were with him [i.e., the emigrants and the helpers], and if it becomes very crowded, a person among you is to make the prostration on the back of his brother." When, he saw some people praying in the street, he said to them: "Pray in the mosque."

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