Salim Abu al-Nadr, who was the freed slave of Umar bin Ubaydullah and his scribe, says: Abdullah bin Abi Awfa (may Allah be pleased with him) wrote a letter at the time of setting out towards the Haruriyyah, in which it was (stated) that on some of the days when the Messenger of Allah (peace and blessings be upon him) fought the enemy, he waited until the sun had declined, then addressed the people, saying: "O people, do not wish to meet the enemy, and ask Allah for well-being; and when you meet the enemy, be steadfast, and know that Paradise is beneath the shade of swords." Then he said: 'Allahumma munzil al-kitab, wa mujriya al-sahab, wa hazim al-ahzab, ihzimhum wansurna alayhim' (O Allah, Revealer of the Book, Mover of the clouds, Defeater of the confederates, defeat them and help us against them). The narrator says that Abu al-Nadr said: It has reached us that the Prophet (peace and blessings be upon him) made a similar supplication: 'Anta rabbuna wa rabbuhum, wa nahnu abiduka wa hum abiduka, wa nawasina wa nawasihim bi-yadika, fahzimhum wansurna alayhim' (You are our Lord and their Lord; we are Your servants and they are Your servants; our forelocks and their forelocks are in Your hand; so defeat them and help us against them).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18462