Qais b. ‘Abbad said : I said to ‘All (Allah be pleased with him) : Tell me about this march of yours. Is this an order that the Messenger of Allah (May peace be upon him) had given you, or is this your opinion that you have? He said: The Messenger of Allah (May peace be upon him) did not give me any order; but this is an opinion that I have.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْمَاعِيلُ بْنُ إِبْرَاهِيمَ الْهُذَلِيُّ، حَدَّثَنَا ابْنُ عُلَيَّةَ، عَنْ يُونُسَ، عَنِ الْحَسَنِ، عَنْ قَيْسِ بْنِ عُبَادٍ، قَالَ قُلْتُ لِعَلِيٍّ رضى الله عنه أَخْبِرْنَا عَنْ مَسِيرِكَ هَذَا أَعَهْدٌ عَهِدَهُ إِلَيْكَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَمْ رَأْىٌ رَأَيْتَهُ فَقَالَ مَا عَهِدَ إِلَىَّ رَسُولُ اللَّهِ صلى الله عليه وسلم بِشَىْءٍ وَلَكِنَّهُ رَأْىٌ رَأَيْتُهُ .
sunnah.com