When Umar consulted him about taking part in the march towards the Roman Empire
1
ومن كلام له (عليه السلام) وقد شاوره عمر بن الخطاب في الخروج إلى غزوالروم
Delivered when Caliph ‘Umar ibn al-Khaṭṭāb consulted [1] Amīr al-mu’minīn about himself, taking part in the march towards Rome (Byzantine Empire).
2
وَقَدْ تَوَكَّلَ اللهُ لاِهْلِ هَذا الدِّينِ بِإِعْزَازِ الْحَوْزَةِ، وَسَتْرِ الْعَوْرَةِ، وَالَّذِي نَصَرَهُمْ وَهُمْ قَلِيلٌ لاَ يَنْتَصِرُونَ، وَمَنَعَهُمْ وَهُمْ قَلِيلٌ لاَ يَمتَنِعُونَ، حَيٌّ لاَ يَمُوتُ. إِنَّكَ مَتَى تَسِرْ إِلَى هذَا الْعَدُوِّ بِنَفْسِكَ، فَتَلْقَهُمْ بِشَخْصِكَ فَتُنْكَبْ، لاَ تَكُنْ لِلْمُسْلِمِينَ كَانِفَةٌ دُونَ أَقْصَى بِلاَدِهِمْ، وَلَيْسَ بَعْدَكَ مَرْجِعٌ يَرْجِعُونَ إِلَيْهِ، فَابْعَثْ إِلَيْهِمْ رَجُلاً مِحْرَباً، وَاحْفِزْ مَعَهُ أَهْلَ الْبَلاَءِ وَالنَّصِيحَةِ، فَإِنْ أَظْهَرَ اللهُ فَذَاكَ مَا تُحِبُّ، وَإِنْ تَكُنِ الاْخْرَى، كُنْتَ رِدْءاً للنَّاسِ وَمَثَابَةً لِلْمُسْلِمِينَ.
Allāh has taken upon himself for the followers of this religion the strengthening of boundaries and hiding of the secret places. Allāh helped them when they were few and could not protect themselves. He is living and will not die. If you will yourself proceed towards the enemy and clash with them and fall into some trouble, there will be no place of refuge for the Muslims other than their remote cities, nor any place they would return to. Therefore, you should send there an experienced man and send with hire people of good performance who are well-intentioned. If Allāh grants you victory, then this is what you want. If it is otherwise, you would serve as a support for the people and a returning place for the Muslims.