What he said facing an army
He waited until the sun had passed its height before saying anything.
اللَّهُمَّ مُنْزِلَ الْكِتَابِ، وَمُجْرِيَ السَّحَابِ، وَهَازِمَ الأَحْزَابِ، اهْزِمْهُمْ وَانْصُرْنَا عَلَيْهِمْ
Allāhumma munzila-l-kitāb, wa mujriya-s-saḥāb, wa hāzima-l-aḥzāb, ihzimhum wa-nṣurnā ʿalayhim
“O Allah, Revealer of the Book, Mover of the clouds, Defeater of the armies, defeat them and help us against them.”
Where this comes from
He stood up among the people first and told them not to wish for a meeting with the enemy, and to ask Allah for wellbeing. Only after that did he say this. The three titles rhyme in Arabic, and each one names something that moves on its own until it is moved.
The full narration
Narrated Salim Abu An-Nadr:The freed slave of `Umar bin 'Ubaidullah who was `Umar's clerk: `Abdullah bin Abi `Aufa wrote him (i.e. `Umar) a letter that contained the following:-- "Once Allah's Messenger (ﷺ) (during a holy battle), waited till the sun had declined and then he got up among the people and said, "O people! Do not wish to face the enemy (in a battle) and ask Allah to save you (from calamities) but if you should face the enemy, then be patient and let it be known to you that Paradise is under the shades of swords." He then said,, "O Allah! The Revealer of the (Holy) Book, the Mover of the clouds, and Defeater of Al-Ahzab (i.e. the clans of infidels), defeat them infidels and bestow victory upon us
Source
Ṣaḥīḥ al-Bukhārī · 2965
- Agreed upon as authentic (al-Bukhārī / Muslim)