Sahih Bukhari Volume 4, Book 52, Hadith Number 266A.
Narated By Salim Abu An-Nadr : (The freed slave of ‘Umar bin ‘Ubaidullah) I was Umar’s clerk. Once Abdullah bin Abi Aufa wrote a letter to ‘Umar when he proceeded to Al-Haruriya. I read in it that Allah’s Apostle in one of his military expeditions against the enemy, waited till the sun declined and then he got up amongst the people saying, “O people! Do not wish to meet the enemy, and ask Allah for safety, but when you face the enemy, be patient, and remember that Paradise is under the shades of swords.” Then he said, “O Allah, the Revealer of the Holy Book, and the Mover of the clouds and the Defeater of the clans, defeat them, and grant us victory over them.”